staging: iio: push the main buffer chrdev down to the top level.

Sorry all, this one is very invasive, though the driver changes are
just trivial interface fixes. Not all done yet.

V2 - bring the sca3000 with us.
V3 - fix ade7758 bugs in conversion.
V4 - add ad5933

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>