staging:iio: rework of attribute registration.

This set also includes quite a number of bug fixes of particularly
remove functions.

Necessary due to issue pointed out in Bart Van Assche's patch:
docs/driver-model: Document device.groups

V2: Rebase due to patch reordering.
V3: Pull various error fixes and cleanups out into their own patches.

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>