staging:iio:adc: unused header removals.

More headers now not needed after iio_chan_spec conversions.
A couple of drivers were using helpers from adc.h.

Given these were trivial, they are pushed down into the drivers.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>