ALSA: firewire-motu: code refactoring for source detection of sampling clock in v3 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/20210623075941.72562-7-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>