fsl_usdpaa: fix get link status crash

Fix crash observed in VSP while trying to get link status when link is
down from kernel.

Signed-off-by: Rohit Raj <rohit.raj@nxp.com>
DPDK-3239

drivers/staging/fsl_qbman/fsl_usdpaa.c

@@ -1849,9 +1849,13 @@ ioctl_usdpaa_get_link_status(struct usdpaa_ioctl_link_status_args *input)
 	if (net_dev == NULL)
 		return -ENODEV;
 
-	input->link_status = netif_carrier_ok(net_dev);
-	input->link_autoneg = net_dev->phydev->autoneg;
-	input->link_duplex = net_dev->phydev->duplex;
+	if (net_dev->phydev == NULL) { /* Interface is down from kernel */
+		input->link_status = ETH_LINK_DOWN;
+	} else {
+		input->link_status = netif_carrier_ok(net_dev);
+		input->link_autoneg = net_dev->phydev->autoneg;
+		input->link_duplex = net_dev->phydev->duplex;
+	}
 
 	if (input->link_status)
 		input->link_speed = net_dev->phydev->speed;