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

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>

drivers/iio/dac/ad5592r-base.c

@@ -413,7 +413,7 @@ static int ad5592r_read_raw(struct iio_dev *iio_dev,
 			s64 tmp = *val * (3767897513LL / 25LL);
 			*val = div_s64_rem(tmp, 1000000000LL, val2);
 
-			ret = IIO_VAL_INT_PLUS_MICRO;
+			return IIO_VAL_INT_PLUS_MICRO;
 		} else {
 			int mult;
 
@@ -444,7 +444,7 @@ static int ad5592r_read_raw(struct iio_dev *iio_dev,
 		ret =  IIO_VAL_INT;
 		break;
 	default:
-		ret = -EINVAL;
+		return -EINVAL;
 	}
 
 unlock: