HID: hid-multitouch: ensure slots are initialized

In case a device does not provide the feature "Maximum Contact Count",
or set it at 0, the maxcontacts field may be at 0 while calling
input_mt_init_slots.

This patch ensures that hid-multitouch will allways report
ABS_MT_SLOT and ABS_MT_TRACKING_ID to the user space.

This corrects a bug found with some Ilitek devices that has been
integrated in 3.0-rc0.

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