iio: adc: meson-saradc: unlock on error in meson_sar_adc_lock()

The meson_sar_adc_lock() function is not supposed to hold the
"indio_dev->mlock" on the error path.

Fixes: 3adbf342 ("iio: adc: add a driver for the SAR ADC found in Amlogic Meson SoCs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>