staging:iio:cdc:ad7150: More consistent register and field naming

Add _REG postfix to register addresses to avoid confusion with
fields.  Also add additional field defines and use throughout the
driver in place of magic numbers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-14-jic23@kernel.org