Staging: iio: adis16209: Add _REG postfix for registers

The defined names for registers does not make it very
clear that they are registers and hence, add _REG postfix.
This improves the readability of the code.

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