HID: elan: Query device max_x and max_y value from touchpad

Query the device's max_x and max_y value from the touchpad rather then
hardcoding it. This makes adding support for other USB ids a lot easier.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>