Input: synaptics - allocate 3 slots to keep stability in image sensors

When slowly dropping 1, 2 and then 3 fingers on an image sensor touchpad,
we can see that the first finger gets reassigned a new slot while it did
not move. This is due to the kernel tracking algorithm which can not assign
correctly the 3 touches, being out of slots.

Declaring that we support 3 slots allows to actually forward:
slot 0 -> down, slot 1 -> up, slot 2 -> down

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Henrik Rydberg <rydberg@bitmath.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>