staging: iio: adc: ad799x miscellaneous cleanups

-Simplify by using iio_alloc_pollfunc helper function
-Print in_scale in decimal format.
-Simplify ring pointer dereferences

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>