Input: edt-ft5x06 - ignore touchdown events

The chip may report invalid coordinates on touchdown events, so don't
report the initial touchdown event.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>