ALSA: firewire-tascam: initialize parameters at open of rawmidi character devices

In current design of ALSA driver for TASCAM FireWire series, initialization
of members in asymc midi port structure is done at device probing. Some of
the members should be initialized every time to use rawmidi devices because
they're changed in sequence of transmission for MIDI messages.

This commit adds a new function to initialize them. Invariant parameters
during object lifetime are kept as is.

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