lldp: announce shutdown with a TTL of 0

lldp off and port <n> lldp block went silent and left the neighbour
holding our entry until its TTL ran out, 120 s later. Send one LLDPDU
with a TTL of 0 on the ports that stop, the shutdown form 802.1AB
defines for this, before the transmit mask changes.

rtl837x_lldp.h

@@ -42,6 +42,7 @@
 void lldp_init(void) __banked;
 void lldp_tick(void) __banked;
 void lldp_send(void) __banked;
+void lldp_shutdown(uint16_t ports) __banked;
 void lldp_set_addresses(void) __banked;
 void lldp_set_rtl_wrapper(void) __banked;
 uint8_t lldp_sysname(__xdata uint8_t *p) __banked;