iio: adc: aspeed: clear reference voltage bits before configuring vref

iio: adc: aspeed: clear reference voltage bits before configuring vref

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: 1b5ceb55fec2 ("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>