ALSA: firewire-motu: use macro instead of magic number for clock source in v3 protocol

This commit adds a series of macro for source of sampling clock in
version 3 protocol.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210623075941.72562-8-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>