staging:iio:meter:ade7854: Allocate buffers in state and state with iio_dev.

Requires moving a few things around, but should be no functional changes.

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