staging:iio: rationalization of different buffer implementation hooks.

1) move a generic helper function out of ring_sw. It applies to other buffers as well.
2) Get rid of a lot of left over function definitions.
3) Move all the access functions into static structures.
4) Introduce and use a static structure for the setup functions, preenable etc.

Some driver conversions thanks to Michael Hennerich (pulled out of patches
that would otherwise sit after this).

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>