iio: proximity: as3935: correct IIO_CHAN_INFO_RAW output

IIO_CHAN_INFO_RAW was returning processed data which was incorrect.
This also adds the IIO_CHAN_INFO_SCALE value to convert to a processed value.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>