ASoC: wm8958: Move DSP firmware lock to driver level

The wm8958 driver uses the snd_soc_codec mutex to protect the various
firmware pointers from concurrent assignment. 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>