sound: usb-audio: add MIDI drain callback

When draining, instead of waiting for fifty milliseconds, just wait for
the currently active URBs to complete.  This cuts the usual waiting time
down to one USB frame, or zero in the common case when there is no URB.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>