Check the clock-halted flag every time the clock is read, not just once

at startup.  The flag stays set until the clock is loaded with good time,
so we need to keep saying the time is invalid until that happens.

sys/dev/iicbus/ds1307.c

@@ -328,6 +328,10 @@ ds1307_gettime(device_t dev, struct timespec *ts)
 		return (error);
 	}
 
+	/* If the clock halted, we don't have good data. */
+	if (data[DS1307_SECS] & DS1307_SECS_CH)
+		return (EINVAL);
+
 	/* If chip is in AM/PM mode remember that. */
 	if (data[DS1307_HOUR] & DS1307_HOUR_USE_AMPM) {
 		sc->sc_use_ampm = true;