staging: iio: adt7316: remove dac vref buffer bypass from adt751x

The option to allow the external vref to bypass the reference buffer is
only available for adt7316/7/8. Remove the attributes for adt751x as
well as the chip->id checks from the show and store functions.

Signed-off-by: Jeremy Fertic <jeremyfertic@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>