staging: iio: cdc: ad7150: create macro for capacitance channels

Create macro for capacitance channels in order to remove the repeated
code and improve its readability.

Signed-off-by: Bárbara Fernandes <barbara.fernandes@usp.br>
Signed-off-by: Wilson Sales <spoonm@spoonm.org>
Co-developed-by: Wilson Sales <spoonm@spoonm.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>