ALSA: firewire-tascam: support drain callback for MIDI playback substream

ALSA driver for TASCAM FireWire series transfers MIDI messages in system
workqueue. In current design of the driver, applications should wait for
sequence of transmission when they close ALSA rawmidi character devices.
However, when considering design of rawmidi interface, it's preferable
to wait in drain ioctl.

This commit adds support for the drain ioctl to wait for the end of
the transmission.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>