stp: correct the timer tick rate (50 Hz, measured)

The timers assumed stp_timers() runs at 64 Hz. It does not: the main loop
idles on the 200 Hz system tick and calls us every fourth pass, i.e.
50 Hz. Measured on hardware - with hello configured to 2 s the BPDUs left
the port 2.560 s apart, exactly the 28 % overshoot the wrong constant
implies, and every other timer (forward delay, max age, tx-hold refill)
was stretched the same way.

Move the constant to the header with the arithmetic spelled out, and use
it in the status page too, which had the 64 hardcoded and therefore aged
the same counters differently than the engine.

httpd/page_impl.c

@@ -643,7 +643,7 @@ void send_stp(void)
 		itoa_html(stp_pp2p[pi_i]);
 		/* designated info: a freshly heard BPDU wins, else we are the
 		 * segment's designated bridge and report our own values */
-		stp_we_root = stp_dbridge[pi_i].mac[5] && stp_bpdu_age[pi_i] < (uint16_t)stp_maxage_s * 64;
+		stp_we_root = stp_dbridge[pi_i].mac[5] && stp_bpdu_age[pi_i] < (uint16_t)stp_maxage_s * STP_HZ;
 		slen += strtox(outbuf + slen, ",\"db\":\"");
 		if (stp_we_root) {
 			pi_prio = stp_dbridge[pi_i].prio; pi_ext = stp_dbridge[pi_i].ext;

rtl837x_stp.c

@@ -101,8 +101,6 @@ __xdata uint16_t stp_tc_while;		/* ticks left to set the TC flag in our BPDUs */
 __xdata uint8_t  stp_i;		/* shared loop iterator (DSEG relief) */
 __xdata uint32_t stp_cost_scratch;
 
-/* stp_timers() runs at ~64 Hz (main loop ~256 Hz / (STP_TICK_DIVIDER+1)) */
-#define STP_HZ		64
 #define STP_EDGE_DELAY	(3 * STP_HZ)	/* auto-edge: forward after 3 s without BPDU */
 
 #define AUTO_COST	20000UL		/* path cost used when stp_pcost == 0 (1G default) */

rtl837x_stp.h

@@ -9,6 +9,13 @@ void stp_off(void) __banked;
 void stp_parse(void) __banked __reentrant;	/* "stp ..." CLI handler (cmd_parser delegates here) */
 void stp_defaults(void) __banked;	/* boot init: 802.1D/w default configuration */
 
+/* Tick rate of stp_timers(): the main loop idles on the 200 Hz system tick
+ * and rtlplayground.c calls us every (STP_TICK_DIVIDER + 1) = 4th pass.
+ * Measured on hardware: hello 2 s produced BPDUs exactly 2.560 s apart with
+ * the previous value of 64, i.e. 20 ms per tick - every configured timer ran
+ * 28 % long. Shared with the web UI, which ages the same counters. */
+#define STP_HZ 50
+
 /* Bridge identifier as carried in a BPDU (priority, extension, MAC). */
 struct bridge {
 	uint8_t prio;