iio: adc: ad7780: define/use own IIO channel macros

This change gets rid of the AD_SD_*_CHANNEL macros in favor of defining
it's own. The ad7780 is quite simpler than it's other Sigma-Delta brothers.

It turned out that centralizing the AD_SD_*_CHANNEL macros doesn't scale
too well, especially with some more complicated drivers. Some of the
variations in the more complicated drivers require new macros, and that way
things can become harder to maintain.

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