ASoC: wm8962: Move DSP enable lock to the driver level

The wm8962 uses the snd_soc_codec mutex to protect the wm8962_dsp2_ena_put()
function from concurrent execution. This patch moves that 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>