Staging: iio/adc/ad7150: release lock on error

The error_ret label should have been before the mutex_unlock().  It's
a typo.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>