dsa: Change dsa_uses_{dsa, trailer}_tags() into inline functions

eth_type_trans() will use these functions if DSA is enabled, which
blocks building DSA as a module.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 999bb26..63721a6 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1080,7 +1080,7 @@
 	struct vlan_group __rcu	*vlgrp;		/* VLAN group */
 #endif
 #ifdef CONFIG_NET_DSA
-	void			*dsa_ptr;	/* dsa specific data */
+	struct dsa_switch_tree	*dsa_ptr;	/* dsa specific data */
 #endif
 	void 			*atalk_ptr;	/* AppleTalk link 	*/
 	struct in_device __rcu	*ip_ptr;	/* IPv4 specific data	*/