HID: multitouch: remove .maxcontacts field for eGalax

Some eGalax devices are 4 or 5 fingers touches, wereas others are 2.
This patch removes the limit in which all eGalax presents 2 touches.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>