alps.c hardcodes 5 slots in the kernel but some devices only provide 2 slots
plus BTN_TOOL_TRIPLETAP, etc. Fix this by counting active slots and when the
fake finger count exceeds the active slots but is still less than the number
of slots, adjust the slots themselves downwards.

And because the new test device messes with our slot count assumptions for the
various tests hardcode that one device to return 2 slots.

Fixes #408



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>