Clarify usb_powered() and fix some code.

Either by mistake or because its meaning changed, usb_powered() doesn't mean
what the name suggest, so clarify its meaning by renaming it to usb_powered_only.
So use of usb_powered() are replaced by usb_inserted() when it makes more sense.

Change-Id: I112887e2d8560e84587bee5f55c826dde8c806d8
Reviewed-on: http://gerrit.rockbox.org/1097
Reviewed-by: Amaury Pouly <amaury.pouly@gmail.com>

firmware/usb.c

@@ -769,7 +769,7 @@ int usb_release_exclusive_storage(void)
 }
 
 #ifdef HAVE_USB_POWER
-bool usb_powered(void)
+bool usb_powered_only(void)
 {
     return usb_state == USB_POWERED;
 }