Don't treat devices with (ABS_MT_SLOT - 1) as multitouch devices

Some devices (PS3 sixaxis controller) merely have a bunch of axes, without the
semantic information that linux/input.h requires. For those, the ABS_MT range
may be merely another axis, not the special range that we need to treat it
with.

Use a simple heuristic: if ABS_MT_SLOT - 1 is enabled, don't treat ABS_MT as
multitouch axes. The ABS_MT_SLOT - 1 axis is not used for a real axis.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>