ALSA: firewire-lib: code refactoring to refer the same frame count per period in domain structure

The number of PCM frame per period is common between PCM substreams
handled in AMDTP stream in AMDTP domain.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210518130048.146596-2-o-takashi@sakamocchi.jp


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