nfp: add port from netdev validation for EEPROM access

Setting of the port flag `NFP_PORT_CHANGED`, introduced
to ensure the correct reading of EEPROM data, causes a
fatal kernel NULL pointer dereference in cases where
the target netdev type cannot be determined.

Add validation of port struct pointer before attempting
to set the `NFP_PORT_CHANGED` flag. Return that operation
is not supported if the netdev type cannot be determined.

Fixes: 4ae97ca ("nfp: ethtool: fix the display error of `ethtool -m DEVNAME`")
Signed-off-by: Jaco Coetzee <jaco.coetzee@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/netronome/nfp/nfp_net_ethtool.c

@@ -1432,6 +1432,9 @@ nfp_port_get_module_info(struct net_device *netdev,
 	u8 data;
 
 	port = nfp_port_from_netdev(netdev);
+	if (!port)
+		return -EOPNOTSUPP;
+
 	/* update port state to get latest interface */
 	set_bit(NFP_PORT_CHANGED, &port->flags);
 	eth_port = nfp_port_get_eth_port(port);