staging: iio: accel: adis16240: Improve readability on write_raw function

Replace shift and minus operation by GENMASK macro and remove the local
variables used to store intermediate data.

Signed-off-by: Rodrigo Ribeiro Carvalho <rodrigorsdc@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>