staging:iio:buffering move the copy to user on rip down into implementations

The current interface is not as adaptable as it should be. Moving
this complexity into the implementations makes it easier to add
new implementations.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Tested-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>