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 <[e-mail]>
Acked-by: Hans de Goede <[e-mail]>
Acked-by: Henrik Rydberg <[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/mouse/synaptics.c

···
 					ABS_MT_POSITION_Y);
 		/* Image sensors can report per-contact pressure */
 		input_set_abs_params(dev, ABS_MT_PRESSURE, 0, 255, 0, 0);
-		input_mt_init_slots(dev, 2, INPUT_MT_POINTER | INPUT_MT_TRACK);
+		input_mt_init_slots(dev, 3, INPUT_MT_POINTER | INPUT_MT_TRACK);
 
 		/* Image sensors can signal 4 and 5 finger clicks */
 		__set_bit(BTN_TOOL_QUADTAP, dev->keybit);