Input: elantech - force resolution of 31 u/mm

All Elantech touchpads pre-v4 with dynamic resolution queries have a fixed
resolution of 800dpi -> 31.49 units/mm. Set this statically, so userspace
does not have to guess.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>