net: mv643xx_eth: potential NULL dereference in probe()

We assume that "mp->phy" can be NULL a couple lines before the
dereference.

Fixes: 1cce16d37d0f ('net: mv643xx_eth: Add missing phy_addr_set in DT mode')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/marvell/mv643xx_eth.c b/drivers/net/ethernet/marvell/mv643xx_eth.c
index 00cd36e..61088a6 100644
--- a/drivers/net/ethernet/marvell/mv643xx_eth.c
+++ b/drivers/net/ethernet/marvell/mv643xx_eth.c
@@ -2890,7 +2890,8 @@
 					 PHY_INTERFACE_MODE_GMII);
 		if (!mp->phy)
 			err = -ENODEV;
-		phy_addr_set(mp, mp->phy->addr);
+		else
+			phy_addr_set(mp, mp->phy->addr);
 	} else if (pd->phy_addr != MV643XX_ETH_PHY_NONE) {
 		mp->phy = phy_scan(mp, pd->phy_addr);