ASoC: wm8903: Move the deemph lock to the driver level

The wm8903 uses the snd_soc_codec mutex to protect its deemph settings from
concurrent access. This patch moves this lock to the driver level. This will
allow us to eventually remove the snd_soc_codec mutex.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>