ALSA: firewire-tascam: add support for MIDI functionality

In former commits, this driver got functionalities to transfer/receive
MIDI messages to/from TASCAM FireWire series.

This commit adds some ALSA MIDI ports to enable userspace applications
to use the functionalities.

I note that this commit doesn't support virtual MIDI ports which console
models support. A physical controls can be assigned to a certain MIDI
ports including physical and virtual. But the way is not clear.

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