Input: alps - allow up to 2 invalid packets without resetting device

On some Dell Latitude laptops ALPS device or Dell EC send one invalid byte
in 6 bytes ALPS packet. In this case psmouse driver enter out of sync
state. It looks like that all other bytes in packets are valid and also
device working properly. So there is no need to do full device reset, just
need to wait for byte which match condition for first byte (start of
packet). Because ALPS packets are bigger (6 or 8 bytes) default limit is
small.

This patch increase number of invalid bytes to size of 2 ALPS packets which
psmouse driver can drop before do full reset.

Resetting ALPS devices take some time and when doing reset on some Dell
laptops touchpad, trackstick and also keyboard do not respond. So it is
better to do it only if really necessary.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/alps.c

@@ -2395,6 +2395,9 @@ int alps_init(struct psmouse *psmouse)
 	/* We are having trouble resyncing ALPS touchpads so disable it for now */
 	psmouse->resync_time = 0;
 
+	/* Allow 2 invalid packets without resetting device */
+	psmouse->resetafter = psmouse->pktsize * 2;
+
 	return 0;
 
 init_fail: