ALSA: seq: virmidi: Add a drain operation

ALSA: seq: virmidi: Add a drain operation

If a driver does not supply a drain operation for outputs, a default code
path will execute msleep(50). Especially for a virtual midi device
this severely limmits the throughput.

This implementation for the virtual midi driver simply flushes the output
workqueue.

Signed-off-by: Stefan Sauer <st_kost@gmx.de>
Link: https://lore.kernel.org/r/20220106124145.17254-1-st_kost@gmx.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>