Input: MT - initialize slots to unused

For MT slots, the ABS_MT_TRACKING_ID determines whether a slot is in use,
but currently leaves initialization up to the drivers. This patch sets the
slot state to unused upon creation.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>