Add USB Audio 1.0 support

Original commit credit to Amaury Pouly, Moshe Piekarski
Pushed across the finish line by Dana Conrad

To enable, see setting under General Settings --> System --> USB-DAC.
On devices with few endpoints, this may not work while HID and/or
mass storage is enabled.

Adds new dedicated mixer channel.

setting usb-dac can have values:
- never (0)
- always (1)
- while_charge_only (2)
- while_mass_storage (3)

Relevant devices are DWC2 and ARC usb controller devices. That being:
x1000 Native targets (m3k, erosqnative, q1, others...?),
sansac200, creativezenxfi2, vibe500, ipodmini2g,
ipod4g, creativezenxfi, creativezenxfi3, sansaview, ipodcolor,
creativezenxfistyle, samsungypz5, sansafuzeplus, iriverh10_5gb,
tatungtpj1022, gigabeats, faketarget, samsungyh820, gogearhdd1630, samsungyh925, ipodmini1g, ipodvideo, creativezenmozaic, sonynwze370, creativezen, gogearsa9200, gogearhdd6330, sonynwze360, sansae200, mrobe100, iriverh10, creativezenv, ipodnano1g, samsungyh920

USB Driver-wise, it should be noted that this patch requires some
slight changes:
- proper blocking on control OUT transfers, to make sure the data is
  received *before* using it, the usb_core should probably use that too
- drivers can now support interface alternate settings
- drivers can be notified of completion by a new fast handler, which
  is called directly from the driver; this is is necessary for
  isochronous transfers because going through the usb queue is way too
  slow

Designware changes:

- enable for USBOTG_DESIGNWARE
- set maxpacketsize to 1023 for ISO endpoints

Change-Id: I570871884a4e4820b4312b203b07701f06ecacc6

firmware/usb.c

@@ -97,6 +97,9 @@ static bool exclusive_storage_access = false;
 #ifdef USB_ENABLE_HID
 static bool usb_hid = true;
 #endif
+#ifdef USB_ENABLE_AUDIO
+static int usb_audio = 0;
+#endif
 
 #ifdef USB_FULL_INIT
 static bool usb_host_present = false;
@@ -194,6 +197,10 @@ static inline void usb_handle_hotswap(long id)
 
 static inline bool usb_configure_drivers(int for_state)
 {
+#ifdef USB_ENABLE_AUDIO
+    // FIXME: doesn't seem to get set when loaded at boot...
+    usb_audio = global_settings.usb_audio;
+#endif
     switch(for_state)
     {
     case USB_POWERED:
@@ -207,6 +214,9 @@ static inline bool usb_configure_drivers(int for_state)
         usb_core_enable_driver(USB_DRIVER_HID, true);
 #endif /* USB_ENABLE_CHARGING_ONLY */
 #endif /* USB_ENABLE_HID */
+#ifdef USB_ENABLE_AUDIO
+        usb_core_enable_driver(USB_DRIVER_AUDIO, (usb_audio == 1) || (usb_audio == 2)); // while "always" or "only in charge-only mode"
+#endif /* USB_ENABLE_AUDIO */
 
 #ifdef USB_ENABLE_CHARGING_ONLY
         usb_core_enable_driver(USB_DRIVER_CHARGING_ONLY, true);
@@ -224,6 +234,9 @@ static inline bool usb_configure_drivers(int for_state)
 #ifdef USB_ENABLE_HID
         usb_core_enable_driver(USB_DRIVER_HID, usb_hid);
 #endif
+#ifdef USB_ENABLE_AUDIO
+        usb_core_enable_driver(USB_DRIVER_AUDIO, (usb_audio == 1) || (usb_audio == 3)); // while "always" or "only in mass-storage mode"
+#endif /* USB_ENABLE_AUDIO */
 #ifdef USB_ENABLE_CHARGING_ONLY
         usb_core_enable_driver(USB_DRIVER_CHARGING_ONLY, false);
 #endif
@@ -845,6 +858,13 @@ void usb_set_hid(bool enable)
 }
 #endif /* USB_ENABLE_HID */
 
+#ifdef USB_ENABLE_AUDIO
+void usb_set_audio(int value)
+{
+    usb_audio = value;
+}
+#endif /* USB_ENABLE_AUDIO */
+
 #ifdef HAVE_USB_POWER
 bool usb_powered_only(void)
 {