ALSA: firewire-motu: refactoring protocol v2 for fetching mode switch

This commit splits the method to switch fetching mode 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/20200519111641.123211-15-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>