ALSA: firewire-tascam: use fixed-length array for message cache to async midi port

ALSA driver for TASCAM FireWire series internally allocates 4 byte buffer
for asynchronous transaction to transfer MIDI messages. However, the buffer
can be allocated with memory object of parent structure.

This commit adds 4 byte array as a member of the structure and obsoletes
the redundant allocation. This is deallocated with memory object of parent
structure.

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