iio: adc: ad7298: check regulator for null in ad7298_get_ref_voltage()

'st->ext_ref' & 'st->reg' are both non-zero/non-null at the same time, so
logically the code isn't broken.
But it is more correct to check that 'st->reg' is non-null, since we make
sure that the regulator is NULL (in probe) in case one isn't defined.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20201127094038.91714-2-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>