iio: adc: ad7791: define/use own IIO channel macros

This driver seems to use most of the AD_SD_*_CHANNEL. This change will move
them in the driver. The intent is that if a new part comes along which
would require tweaks per IIO channel, these should be doable in the driver.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>