staging:iio: pull out demux cleanup for a particular buffer.

This will come in handy again when we move to multiple buffers
so lets pull it out into a little utility function now.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>