hid: egalax: Add event filtering

Use estimated signal-to-noise ratios to reduce noise and limit the
amount of events emitted.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Cc: Stephane Chatty <chatty@enac.fr>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>