rtc: fix inverted resolution check

The current code in clock_register checks if the newly added clock has a
resolution value higher than the current one in order to make it the
default, which is wrong. Clocks with a lower resolution value should be
better than ones with a higher resolution value, in fact with the current
code FreeBSD is always selecting the worse clock.

Reviewed by:		kib jhb jkim
Sponsored by:		Citrix Systems R&D
MFC after:		2 weeks
Differential revision:	https://reviews.freebsd.org/D6185

sys/kern/subr_rtc.c

@@ -84,7 +84,7 @@ clock_register(device_t dev, long res)	/* res has units of microseconds */
 {
 
 	if (clock_dev != NULL) {
-		if (clock_res > res) {
+		if (clock_res <= res) {
 			if (bootverbose)
 				device_printf(dev, "not installed as "
 				    "time-of-day clock: clock %s has higher "