The recently added API that exposes firmware mixer controls to the
kernel is missing cache handling and all writes bypass the cache, this
obviously causes the cache to get out of sync with the hardware. Factor
out the cache handling into two new helper functions and call those from
both the normal ALSA control handlers and the new kernel API.

Fixes: eb65ccdb ("ASoC: wm_adsp: Expose mixer control API")
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20200114161841.451-1-ckeepax@opensource.cirrus.comSigned-off-by: Mark Brown <broonie@kernel.org>