iio: adc: modify NPCM ADC read reference voltage

Checking if regulator is valid before reading
NPCM ADC regulator voltage to avoid system crash
in a case the regulator is not valid.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>