Input: alps - use standard contact tracking instead of DIY

Input: alps - use standard contact tracking instead of DIY

When there are 2 fingers on the pad we don't know which one is which, so
use input_mt_assign_slots to make sure the right set of coordinates ends
up in the right slot.

Besides ensuring things end up in the right slot, this also results in a
nice cleanup, since sync_frame also handles non mt position and btn_touch
reporting.

Signed-off-by: Hans de Goede <[email protected]>
Signed-off-by: Dmitry Torokhov <[email protected]>