staging: iio: adt7316: allow adt751x to use internal vref for all dacs

With adt7516/7/9, internal vref is available for dacs a and b, dacs c and
d, or all dacs. The driver doesn't currently support internal vref for all
dacs. Change the else if to an if so both bits are checked rather than
just one or the other.

Signed-off-by: Jeremy Fertic <jeremyfertic@gmail.com>
Fixes: 35f6b6b8 ("staging: iio: new ADT7316/7/8 and ADT7516/7/9 driver")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>