ASoC: dapm: Add missing mutex unlock

The is a missing mutex unlock on the error path in
snd_soc_dapm_get_enum_double. This was introduced in commit
561ed680

 ("ASoC: dapm: Add support for autodisable mux controls").
This patch adds the missing unlock.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>