mac802154: sparse warnings: make symbols static

Make symbols static to avoid the following warning shown up
by sparse:

    warning: symbol ... was not declared. Should it be static?

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/mac802154/mib.c b/net/mac802154/mib.c
index 5c66b8f..f47781a 100644
--- a/net/mac802154/mib.c
+++ b/net/mac802154/mib.c
@@ -39,7 +39,7 @@
 	unsigned long changed;
 };
 
-struct mac802154_priv *mac802154_slave_get_priv(struct net_device *dev)
+static struct mac802154_priv *mac802154_slave_get_priv(struct net_device *dev)
 {
 	struct mac802154_sub_if_data *priv = netdev_priv(dev);