ASoC: wm_adsp: Add locking to DSP firmware controls

Locking is currently missing from the DSP firmware controls, which can
lead to some race conditions if the controls are accessed as the DSP
powers up or down. This patch adds them to the new power lock.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>