staging:iio:adc:ad799x stop using IIO_CHAN macro.

Preparation for moving driver out of staging.
That macro is a nightmare to maintain so it is going away.
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>