iio: frequency: ad9832: Use FIELD_PREP macro to set bit fields

Use bitfield and bitmask macros to clearly specify AD9832 SPI
command fields to make register write code more readable.

Suggested-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Siddharth Menon <simeddon@gmail.com>
Link: https://patch.msgid.link/20250416140259.13431-1-simeddon@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>