iio:accel:kxsd9 fix missing mutex unlock

This will leave a lock held after reading from the device, preventing
any further reads.
Signed-off-by: NFrank Zago <frank@zago.net>
Cc: stable@vger.kernel.org
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/iio/accel/kxsd9.c

@@ -112,9 +112,10 @@ static int kxsd9_read(struct iio_dev *indio_dev, u8 address)
 	mutex_lock(&st->buf_lock);
 	st->tx[0] = KXSD9_READ(address);
 	ret = spi_sync_transfer(st->us, xfers, ARRAY_SIZE(xfers));
-	if (ret)
-		return ret;
-	return (((u16)(st->rx[0])) << 8) | (st->rx[1] & 0xF0);
+	if (!ret)
+		ret = (((u16)(st->rx[0])) << 8) | (st->rx[1] & 0xF0);
+	mutex_unlock(&st->buf_lock);
+	return ret;
 }
 
 static IIO_CONST_ATTR(accel_scale_available,