caif_dev: fix sparse warnings for caif_flow_cb

This fixed the following sparse warning:
net/caif/caif_dev.c:121:6: warning: symbol 'caif_flow_cb' was not
declared. Should it be static?
Signed-off-by: Silviu-Mihai Popescu <silviupopescu1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/caif/caif_dev.c

@@ -118,7 +118,7 @@ static struct caif_device_entry *caif_get(struct net_device *dev)
 	return NULL;
 }
 
-void caif_flow_cb(struct sk_buff *skb)
+static void caif_flow_cb(struct sk_buff *skb)
 {
 	struct caif_device_entry *caifd;
 	void (*dtor)(struct sk_buff *skb) = NULL;