ALSA: firewire-motu: refactoring protocol v2 for clock source getter

This commit splits the method to get clock source for protocol
version 2 so that model-dependent operations are explicitly defined.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20200519111641.123211-13-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>