Initialise `restartcnt' in the newly malloc'd usbd_port structure,

as otherwise the junk it contains may cause uhub_explore to give
up without ever trying to restart the port. This fixes the following
errors I was seeing with a VIA UHCI controller:

	uhub0: port error, restarting port 1
	uhub0: port error, giving up port 1

sys/dev/usb/uhub.c

@@ -308,6 +308,7 @@ USB_ATTACH(uhub)
 			up->power = USB_MAX_POWER;
 		else
 			up->power = USB_MIN_POWER;
+		up->restartcnt = 0;
 	}
 
 	/* XXX should check for none, individual, or ganged power? */