iio: addac: ad74413r: correct comparator gpio getters mask usage

iio: addac: ad74413r: correct comparator gpio getters mask usage

The value of the GPIOs is currently altered using offsets rather
than masks. Make use of __assign_bit and the BIT macro to turn
the offsets into masks.

Fixes: fea251b6a5db ("iio: addac: add AD74413R driver")
Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220111074703.3677392-2-cosmin.tanislav@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>