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

commit 10bfe7cc1739c22f0aa296b39e53f61e9e3f4d99 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>