ASoC: pcm512x: Fix a double unlock in pcm512x_digital_mute()

We accidentally call mutex_unlock(&pcm512x->mutex); twice in a row.

I re-wrote the error handling to use "goto unlock;" instead of returning
directly.  Hopefully, it makes the code a little simpler.

Fixes: 3500f1c5 ("ASoC: pcm512x: Implement the digital_mute interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviwed-by: Dimitris Papavasiliou <dpapavas@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>