sfc: Change BUG_ON to WARN_ON and recovery code.

Pre-emptively fix a checkpatch warning in a subsequent patch.
Signed-off-by: Jonathan Cooper <jonathan.s.cooper@amd.com>
Co-developed-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/efx.c

@@ -774,7 +774,8 @@ static void efx_unregister_netdev(struct efx_nic *efx)
 	if (!efx->net_dev)
 		return;
 
-	BUG_ON(netdev_priv(efx->net_dev) != efx);
+	if (WARN_ON(netdev_priv(efx->net_dev) != efx))
+		return;
 
 	if (efx_dev_registered(efx)) {
 		strlcpy(efx->name, pci_name(efx->pci_dev), sizeof(efx->name));