liquidio: silence a locking static checker warning

Presumably we never hit this return, but static checkers complain that
we need to unlock so we may as well fix that.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Acked-by: NFelix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/cavium/liquidio/octeon_mailbox.c

@@ -313,6 +313,7 @@ int octeon_mbox_process_message(struct octeon_mbox *mbox)
 		return 0;
 	}
 
+	spin_unlock_irqrestore(&mbox->lock, flags);
 	WARN_ON(1);
 
 	return 0;