Ensures the reference voltage bits are cleared in the ADC engine
control register before configuring the voltage reference. This
avoids potential misconfigurations caused by residual bits.
Fixes: 1b5ceb55fe ("iio: adc: aspeed: Support ast2600 adc.")
Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>