staging:iio:resolver:ad2s1210 general driver cleanup.

Note I haven't made any changes to the userspace interface as yet.
This is all about cleaning up what was actually there (handling
all errors etc).

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>