iio: dac: ad5592r: fix unbalanced mutex unlocks in ad5592r_read_raw()

commit 65afb0932a81c1de719ceee0db0b276094b10ac8 upstream.

There are 2 exit paths where the lock isn't held, but try to unlock the
mutex when exiting. In these places we should just return from the
function.

A neater approach would be to cleanup the ad5592r_read_raw(), but that
would make this patch more difficult to backport to stable versions.

Fixes 56ca9db8: ("iio: dac: Add support for the AD5592R/AD5593R ADCs/DACs")
Reported-by: Charles Stanhope <charles.stanhope@gmail.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>