lldp: time the transmit interval from the system tick

lldp_tick counted main-loop passes, so the 6000 meant for 30 s at
SYS_TICK_HZ ran at whatever rate the loop happened to have, and the
interval moved with httpd and SFP load. Step once per second from the
tick counter like the STP timers do, count seconds to the interval and
derive the TTL from it with the 802.1AB hold multiplier of 4.

rtl837x_lldp.c

@@ -13,24 +13,29 @@
 extern __xdata bool lldp_enabled;
 extern __code const struct machine machine;
 
-__xdata uint16_t lldp_ticks;
+extern volatile __xdata uint32_t ticks;
+
+__xdata uint8_t lldp_tick_last;
+__xdata uint8_t lldp_seconds;
 __xdata uint16_t lldp_logical_port_status = 0xffff;
 
 void lldp_init(void) __banked
 {
-    lldp_ticks = 0;
+    lldp_tick_last = (uint8_t)ticks;
+    lldp_seconds = 0;
 }
 
 void lldp_tick(void) __banked
 {
-    lldp_ticks++;
-
-    if (lldp_ticks < LLDP_TX_INTERVAL_TICKS)
+    if (!TICKS_DUE(lldp_tick_last, SYS_TICK_HZ))
         return;
+    lldp_tick_last += SYS_TICK_HZ;
 
-    lldp_ticks = 0;
+    if (++lldp_seconds < LLDP_TX_INTERVAL_S)
+        return;
+    lldp_seconds = 0;
 
-    if(lldp_enabled == 1)
+    if (lldp_enabled)
         lldp_send();
 }