ALSA: firewire-digi00x: share PCM buffer size for both direction

This commit allows ALSA firewire-digi00x driver to share PCM buffer
size for both capture and playback PCM substream. When AMDTP domain
starts for one of the PCM substream, buffer size of the PCM substream
is stores to AMDTP domain structure. Some AMDTP streams have already
run with the buffer size when another PCM substream starts, therefore
the PCM substream has a constraint to its buffer size.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191017155424.885-7-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>