net: dsa: sja1105: fix displaced ethtool statistics counters

Port counters with no name (aka
sja1105_port_counters[__SJA1105_COUNTER_UNUSED]) are skipped when
reporting sja1105_get_sset_count(), but are not skipped during
sja1105_get_strings() and sja1105_get_ethtool_stats().

As a consequence, the first reported counter has an empty name and a
bogus value (reads from area 0, aka MAC, from offset 0, bits start:end
0:0). Also, the last counter (N_NOT_REACH on E/T, N_RX_BCAST on P/Q/R/S)
gets pushed out of the statistics counters that get shown.

Skip __SJA1105_COUNTER_UNUSED consistently, so that the bogus counter
with an empty name disappears, and in its place appears a valid counter.

Fixes: 039b167 ("net: dsa: sja1105: don't use burst SPI reads for port statistics")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250318115716.2124395-2-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

drivers/net/dsa/sja1105/sja1105_ethtool.c

@@ -571,6 +571,9 @@ void sja1105_get_ethtool_stats(struct dsa_switch *ds, int port, u64 *data)
 		max_ctr = __MAX_SJA1105PQRS_PORT_COUNTER;
 
 	for (i = 0; i < max_ctr; i++) {
+		if (!strlen(sja1105_port_counters[i].name))
+			continue;
+
 		rc = sja1105_port_counter_read(priv, port, i, &data[k++]);
 		if (rc) {
 			dev_err(ds->dev,
@@ -596,8 +599,12 @@ void sja1105_get_strings(struct dsa_switch *ds, int port,
 	else
 		max_ctr = __MAX_SJA1105PQRS_PORT_COUNTER;
 
-	for (i = 0; i < max_ctr; i++)
+	for (i = 0; i < max_ctr; i++) {
+		if (!strlen(sja1105_port_counters[i].name))
+			continue;
+
 		ethtool_puts(&data, sja1105_port_counters[i].name);
+	}
 }
 
 int sja1105_get_sset_count(struct dsa_switch *ds, int port, int sset)