staging:iio:adc:ad7280a: Use bitfield ops to managed fields in transfers.

The write and two types of read transfer are sufficiently complex that
they benefit from the clarity of using FIELD_PREP() and FIELD_GET().
This also applies to the handling in ad7280_event_handler() so
use a similar approach there as well.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-6-jic23@kernel.org