iio: light: hid-sensor-prox: Fix modifier

Currently in_proximity_(null)_raw is getting presented as raw sysfs
attribute. Same with the scan_elements.
The modifier doesn't apply to this channel.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>