nfp: don't dereference a null nn->eth_port to print a warning

On the case where nn->eth_port is null the warning message
is printing the port by dereferencing this null pointer.
Remove the deference to avoid a crash when printing the
warning message.

Detected by CoverityScan, CID#1426198 ("Dereference after null check")

Fixes: ce22f5a2 ("nfp: separate high level and low level NSP headers")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Acked-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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

@@ -495,8 +495,7 @@ static void nfp_net_refresh_netdevs(struct work_struct *work)
 
 	list_for_each_entry_safe(nn, next, &pf->ports, port_list) {
 		if (!nn->eth_port) {
-			nfp_warn(pf->cpp, "Warning: port %d not present after reconfig\n",
-				 nn->eth_port->eth_index);
+			nfp_warn(pf->cpp, "Warning: port not present after reconfig\n");
 			continue;
 		}
 		if (!nn->eth_port->override_changed)