iio: adc: rockchip_saradc: better prefix for channel constant

As suggested give the current ADC_CHANNEL constant a distinct
and consistent prefix.

Suggested-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>