Reduce default tap_min_queue size for Elan touchpads

Elan hw v.4 touchpads often sends touchpad release packet right after
touchpad touch one. Most probably this happens due to PS/2 limited bandwith.
Reducing of tap_min_queue size to 1 makes multifinger tap detection
more reliable in this case.

Reviewed by:	gonzo
Approved by:	gonzo (mentor)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D10266

sys/dev/atkbdc/psm.c

@@ -6218,6 +6218,9 @@ elantech_init_synaptics(struct psm_softc *sc)
 		/* Disable finger detection pressure threshold */
 		sc->syninfo.min_pressure = 1;
 
+		/* Elans often report double & triple taps as single event */
+		sc->syninfo.tap_min_queue = 1;
+
 		/* Use full area of touchpad */
 		sc->syninfo.margin_top = 0;
 		sc->syninfo.margin_right = 0;