net: dsa: microchip: Don't try to read stats for unused ports

If some of the switch ports were not listed in the device tree, due to
being unused, the ksz_mib_read_work function ended up accessing a NULL
dp->slave pointer and causing an oops. Skip checking statistics for any
unused ports.

Fixes: 7c6ff470 ("net: dsa: microchip: add MIB counter reading support")
Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/microchip/ksz_common.c

@@ -83,6 +83,9 @@ static void ksz_mib_read_work(struct work_struct *work)
 	int i;
 
 	for (i = 0; i < dev->mib_port_cnt; i++) {
+		if (dsa_is_unused_port(dev->ds, i))
+			continue;
+
 		p = &dev->ports[i];
 		mib = &p->mib;
 		mutex_lock(&mib->cnt_mutex);