Input: MT - Specify that ABS_MT_SLOT must have a minimum of 0

This is effectively already in force through input_mt_init_slots, and uinput
too ignores the actual minimum.

Since slots are a kernel-genenerated axis only, non-zero minimums make
little sense and are likely to cause errors. Better to treat a non-zero
minimum as kernel bug if it ever happens.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>

Documentation/input/multi-touch-protocol.txt

@@ -80,6 +80,8 @@ Userspace can detect that a driver can report more total contacts than slots
 by noting that the largest supported BTN_TOOL_*TAP event is larger than the
 total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
 
+The minimum value of the ABS_MT_SLOT axis must be 0.
+
 Protocol Example A
 ------------------