ALSA: rawmidi: A lightweight function to discard pending bytes

For discarding the pending bytes on rawmidi, we process with a loop of
snd_rawmidi_transmit() which is just a waste of CPU power.
Implement a lightweight API function to discard the pending bytes and
the proceed the ring buffer instantly, and use it instead of open
codes.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/midi.c

@@ -1175,8 +1175,7 @@ static void snd_usbmidi_output_trigger(struct snd_rawmidi_substream *substream,
 		if (port->ep->umidi->disconnected) {
 			/* gobble up remaining bytes to prevent wait in
 			 * snd_rawmidi_drain_output */
-			while (!snd_rawmidi_transmit_empty(substream))
-				snd_rawmidi_transmit_ack(substream, 1);
+			snd_rawmidi_proceed(substream);
 			return;
 		}
 		tasklet_schedule(&port->ep->tasklet);