staging:iio: add demux optionally to path from device to buffer

This gives you only what you ask for which is handy
for some devices with weird scan combinations.

Routes all data flow through a core utility function.
That and this demuxing support will be needed to do
demuxing to multiple destinations in kernel.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>