We're getting too many regressions on other devices for this feature and only
ALPS touchpads need it (it's a kernel driver bug). So let's limit this to
those devices only.

For example, synaptics serial touchpads don't keep the fake fingers and slot
states in sync when going from two to three fingers, causing an erroneous slot
downgrade. See
https://gitlab.freedesktop.org/libinput/libinput/issues/434#note_419912
That interferes with this code but fixing it is hard and anyway,
synaptics touchpads don't need the slot count drop.

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