staging: iio: ad7780: set pattern values and masks directly

The AD7780 driver contains status pattern bits designed for checking
whether serial transfers have been correctly performed. Pattern macros
were previously generated through bit fields. This patch sets good
pattern values directly and masks through GENMASK.

Signed-off-by: Renato Lui Geh <renatogeh@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>