stp: warn when an enabled port cannot receive BPDUs

A port set to admit tagged frames only will never see a BPDU, because
delivery rides the forward action and the ingress pipeline drops untagged
frames before the L2 lookup. The failure is silent and looks like a dead
receive path: the port turns edge after three seconds, the bridge elects
itself root, and nothing hints at the ingress setting. Diagnosing exactly
that cost most of a day on a live switch, with the neighbour provably
transmitting the whole time.

stp_setup() now prints one line per affected port, so the hint lands at
"stp on" and at every config replay on boot. The check runs in its own
loop after the MSTP write: port_ingress_filter_get() reads a register
into sfr_data, which the state-building loop above is still using. The
port number in the message is physical, matching what the ingress
command takes.

doc/stp.md explains why this can happen here and not on a normal bridge,
where BPDUs are consumed before any VLAN classification.

stp.rel stays at DSEG 5 with no OSEG and the image at 10498 bytes of
XDATA.

rtl837x_stp.c

@@ -720,6 +720,16 @@ void stp_setup(void) __banked
 
 	print_reg(RTL837X_MSTP_STATES); write_char('\n');
 
+	for (stp_i = machine.min_port; stp_i <= machine.max_port; stp_i++) {
+		if (!(stp_pflags[stp_i] & STP_PF_ENABLED))
+			continue;
+		if (port_ingress_filter_get(stp_i) != VLAN_TAGGED)
+			continue;
+		print_string("STP: port ");
+		write_char('0' + machine.log_to_phys_port[stp_i]);
+		print_string(" admits tagged frames only - BPDUs are untagged and will not arrive\n");
+	}
+
 	/* Seed the carrier bitmap, so turning STP on does not report every
 	 * port that was already down as a fresh topology change. */
 	reg_read_m(RTL837X_REG_LINKS_STS);