net: dsa: fix oops while probing Marvell DSA switches

Fix an oops in dsa_port_phylink_mac_change() caused by a combination
of a20f997 ("net: dsa: Don't instantiate phylink for CPU/DSA
ports unless needed") and the net-dsa-improve-serdes-integration
series of patches 65b7a2c ("Merge branch
'net-dsa-improve-serdes-integration'").

Unable to handle kernel NULL pointer dereference at virtual address 00000124
pgd = c0004000
[00000124] *pgd=00000000
Internal error: Oops: 805 [#1] SMP ARM
Modules linked in: tag_edsa spi_nor mtd xhci_plat_hcd mv88e6xxx(+) xhci_hcd armada_thermal marvell_cesa dsa_core ehci_orion libdes phy_armada38x_comphy at24 mcp3021 sfp evbug spi_orion sff mdio_i2c
CPU: 1 PID: 214 Comm: irq/55-mv88e6xx Not tainted 5.6.0+ torvalds#470
Hardware name: Marvell Armada 380/385 (Device Tree)
PC is at phylink_mac_change+0x10/0x88
LR is at mv88e6352_serdes_irq_status+0x74/0x94 [mv88e6xxx]

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

net/dsa/slave.c

@@ -1585,7 +1585,8 @@ void dsa_port_phylink_mac_change(struct dsa_switch *ds, int port, bool up)
 {
 	const struct dsa_port *dp = dsa_to_port(ds, port);
 
-	phylink_mac_change(dp->pl, up);
+	if (dp->pl)
+		phylink_mac_change(dp->pl, up);
 }
 EXPORT_SYMBOL_GPL(dsa_port_phylink_mac_change);