ASoC: hdmi-codec: re-introduce mutex locking

Replace the bit atomic operations by a mutex to ensure only one dai
at a time is active on the hdmi codec.

This is a follow up on change:
3fcf94ef

 ("ASoC: hdmi-codec: remove reference to the current substream")

Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>