Staging: iio: adis16209: Remove and add some comments and group the definitions

Remove some unnecessay comments and group the control
register and register field macros together.
Some of the register names does not make it's puporse
very clear and hence, add some comments for more
information.
Also there are certain unit based comments which are not
providing sufficient information, so expand those comments.

Signed-off-by: Shreeya Patel <shreeya.patel23498@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>