staging: iio: ad7150: use FIELD_GET and GENMASK

Use the bitfield macro FIELD_GET, and GENMASK to do the shift and mask in
one go. This makes the code more readable than explicit masking followed
by a shift.

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>