ALSA: bebob: use the same size of period for PCM substream in AMDTP streams

ALSA: bebob: use the same size of period for PCM substream in AMDTP streams

In current implementation, when opening a PCM substream, it's needed to
check whether the opposite PCM substream runs. This is to assign
effectual constraints (e.g. sampling rate) to opened PCM substream.

The number of PCM substreams and MIDI substreams on AMDTP streams in
domain is recorded in own structure. Usage of this count is an
alternative of the above check. This is better because the count is
incremented in pcm.hw_params earlier than pcm.trigger.

This idea has one issue because it's incremented for MIDI substreams as
well. In current implementation, for a case that any MIDI substream run
and a PCM substream is going to start, PCM application to start the PCM
substream can decide hardware parameters by restart packet streaming.
Just checking the substream count can brings regression.

Now AMDTP domain structure has a member for the size of PCM period in
PCM substream which starts AMDTP streams in domain. When the value has
zero and the substream count is greater than 1, it means that any MIDI
substream starts AMDTP streams in domain. Usage of the value can resolve
the above issue.

This commit replaces the check with the substream count and the value for
the size of PCM period.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191007110532.30270-11-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>