ALSA: firewire-motu: code refactoring for detection of clock source in v2 protocol

Current implementation of driver has two similar helper functions for
source detection of sampling clock. This commit merges them as a code
refactoring.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210623075941.72562-3-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Iwai <tiwai@suse.de>