ASoC: ab8500-codec: Move control lock to the driver level

The ab8500 driver uses a driver specific lock to protect concurrent access
to some of the control put/get handlers and uses the snd_soc_codec mutex for
some others. This patch updates the driver to consistently use the driver
specific lock for all controls. This will allow us to eventually remove the
snd_soc_codec mutex.

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