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

apps/menus/settings_menu.c

@@ -358,6 +358,9 @@ MENUITEM_SETTING(lineout_onoff, &global_settings.lineout_active, NULL);
 MENUITEM_SETTING(usb_hid, &global_settings.usb_hid, NULL);
 MENUITEM_SETTING(usb_keypad_mode, &global_settings.usb_keypad_mode, NULL);
 #endif
+#ifdef USB_ENABLE_AUDIO
+MENUITEM_SETTING(usb_audio, &global_settings.usb_audio, NULL);
+#endif
 #if defined(USB_ENABLE_STORAGE) && defined(HAVE_MULTIDRIVE)
 MENUITEM_SETTING(usb_skip_first_drive, &global_settings.usb_skip_first_drive, NULL);
 #endif
@@ -454,6 +457,9 @@ MAKE_MENU(system_menu, ID2P(LANG_SYSTEM),
             &usb_hid,
             &usb_keypad_mode,
 #endif
+#ifdef USB_ENABLE_AUDIO
+            &usb_audio,
+#endif
 #if defined(USB_ENABLE_STORAGE) && defined(HAVE_MULTIDRIVE)
             &usb_skip_first_drive,
 #endif