ALSA: firewire-motu: add specification flag for position of flag for MIDI messages

In protocols of MOTU FireWire series, when transferring MIDI messages,
transmitter set existence flag to one byte on first several quadlets. The
position differs depending on protocols and models, however two cases are
confirmed; in 5th byte and 8th byte from MSB side.

This commit adds a series of specification flag to describe them. When
the existence flag is in the 5th byte, SND_MOTU_SPEC_[R|T]X_MIDI_2ND_Q is
used. Else, another set of the flag is used. Here, '_Q' means quadlet.

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