lldp: send the port id as an interface name

The Port ID TLV carried a single digit with the locally assigned
subtype, so a neighbour listed us as port "3". Send "Port 3" with the
interface name subtype instead, the form the port keeps in the CLI and
the web UI.

rtl837x_lldp.c

@@ -76,6 +76,8 @@ void lldp_send(void) __banked __reentrant
     *p++ = LLDP_TYPE(LLDP_PORT_ID_TLV_TYPE);
     *p++ = LLDP_PORT_ID_TLV_LENGTH;
     *p++ = LLDP_PORT_ID_TLV_SUBTYPE;
+    memcpyc(p, LLDP_PORT_ID_PREFIX, LLDP_PORT_ID_PREFIX_LENGTH);
+    p += LLDP_PORT_ID_PREFIX_LENGTH;
     port_position = (p-LLDP_O->payload);
 	*p++ = '0';       // filled in per port below