iio: frequency: ad9523: unlock on error in ad9523_reg_access()

There was a return path which got missed accidentally.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>