ASoC: soc-pcm: serialize BE triggers

When more than one FE is connected to a BE, e.g. in a mixing use case,
the BE can be triggered multiple times when the FE are opened/started
concurrently. This race condition is problematic in the case of
SoundWire BE dailinks, and this is not desirable in a general
case.

This patch relies on the existing BE PCM lock, which takes atomicity into
account. The locking model assumes that all interactions start with
the FE, so that there is no deadlock between FE and BE locks.

Signed-off-by: Takashi Iwai <[email protected]>
[test, checkpatch fix and clarification of commit message by plbossart]
Signed-off-by: Pierre-Louis Bossart <[email protected]>
Reviewed-by: Kai Vehmanen <[email protected]>
Reviewed-by: Bard Liao <[email protected]>
Reviewed-by: Ranjani Sridharan <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mark Brown <[email protected]>

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