Modifying ABS_MT_SLOT has no real effects

Submitted by Peter Hutterer @whot

Assigned to Peter Hutterer @whot

Link to original bug (#104270)

Description

Changing ABS_MT_SLOT on a device doesn't actually change the number of slots it has. dev->num_slots is only updated on libevdev_set_fd() so it's not possible to create a device from scratch with slots.