staging:iio:buffer move setup ops from buffer instance to iio_dev

These callbacks should not be buffer instance specific.
Hence move them out of the buffer.

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