HID: ntrig: add sysfs access to filter parameters

This should make it a little more convenient to tweak the filtering
parameters on the fly.  Also unlike load-time parameters, this provides
independent tuning for each device conntected.

Signed-off-by: Rafi Rubin <rafi@seas.upenn.edu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>