ASoC: wm_adsp: Return an error on write to a disabled volatile control

Volatile controls should only be accessed when the firmware is active,
currently however writes to these controls will succeed, but the data
will be lost, if the firmware is powered down. Update this behaviour such
that an error is returned the same as it is for reads.

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