Input - mt: Fix input_mt_get_slot_by_key

Input - mt: Fix input_mt_get_slot_by_key

The case occurred recently with a touchscreen using twice a slot during a
single EV_SYN event:

E: 0.288415 0000 0000 0000      # ------------ SYN_REPORT (0) ----------
E: 0.296207 0003 002f 0000      # EV_ABS / ABS_MT_SLOT          0
E: 0.296207 0003 0039 -001      # EV_ABS / ABS_MT_TRACKING_ID   -1
E: 0.296207 0003 002f 0001      # EV_ABS / ABS_MT_SLOT          1
E: 0.296207 0003 0035 0908      # EV_ABS / ABS_MT_POSITION_X    908
E: 0.296207 0003 0036 1062      # EV_ABS / ABS_MT_POSITION_Y    1062
E: 0.296207 0003 002f 0000      # EV_ABS / ABS_MT_SLOT          0
E: 0.296207 0003 0039 8787      # EV_ABS / ABS_MT_TRACKING_ID   8787
E: 0.296207 0003 0035 1566      # EV_ABS / ABS_MT_POSITION_X    1566
E: 0.296207 0003 0036 0861      # EV_ABS / ABS_MT_POSITION_Y    861
E: 0.296207 0003 0000 0908      # EV_ABS / ABS_X                908
E: 0.296207 0003 0001 1062      # EV_ABS / ABS_Y                1062
E: 0.296207 0000 0000 0000      # ------------ SYN_REPORT (0) ----------

This occurred because while having already slots 0 and 1 assigned, the
touchscreen sent:

0.293377 Tip Switch: 0 | Contact Id: 0 | X:  539 | Y: 1960 | Contact Count: 3
0.294783 Tip Switch: 1 | Contact Id: 1 | X:  908 | Y: 1062 | Contact Count: 0
0.296187 Tip Switch: 1 | Contact Id: 2 | X: 1566 | Y:  861 | Contact Count: 0

Slot 0 is released correclty, but when we look for Contact ID 2, the slot
0 is then picked up again because it is marked as inactive (trackingID < 0).

This is wrong, and we should not reuse a slot in the same frame.
The test should also check for input_mt_is_used().

In addition, we need to initialize mt->frame to an other value than 0.
With mt->frame being 0, all slots are tags as currently used, and so
input_mt_get_slot_by_key() would return -1 for all requests.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=88903

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>