iio:adc:at91: Relase mutex on error path in at91_adc_read_raw

This issue was reported by the mini_lock.cocci coccinelle semantic patch.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/iio/adc/at91_adc.c

@@ -349,9 +349,11 @@ static int at91_adc_read_raw(struct iio_dev *idev,
 						       st->done,
 						       msecs_to_jiffies(1000));
 		if (ret == 0)
-			return -ETIMEDOUT;
-		else if (ret < 0)
+			ret = -ETIMEDOUT;
+		if (ret < 0) {
+			mutex_unlock(&st->lock);
 			return ret;
+		}
 
 		*val = st->last_value;