ALSA: firewire-motu: minor code refactoring for protocol version 2

This commit adds some helper functions to parse register value for
source of sampling clock and nominal sampling transmission frequency.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191030080644.1704-5-o-takashi@sakamocchi.jp


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