staging:iio:meter remove stubs from ade7759.

General cleanup and use of standard functions to simplfy some spi reads
as well.

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>