ASoC: max98095: Move mutex to the driver level

The max98095 uses the snd_soc_codec mutex to protect against concurrent
access in some of its control put handlers. Move this mutex to the driver
level so we can eventually remove the snd_soc_codec mutex.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>