net: dsa: make unexported dsa_link_touch() static

dsa_link_touch() is not exported, or defined outside of the
file it is in so make it static to avoid the following warning:

net/dsa/dsa2.c:127:17: warning: symbol 'dsa_link_touch' was not declared. Should it be static?
Signed-off-by: NBen Dooks (Codethink) <ben.dooks@codethink.co.uk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -124,7 +124,8 @@ static struct dsa_port *dsa_tree_find_port_by_node(struct dsa_switch_tree *dst,
 	return NULL;
 }
 
-struct dsa_link *dsa_link_touch(struct dsa_port *dp, struct dsa_port *link_dp)
+static struct dsa_link *dsa_link_touch(struct dsa_port *dp,
+				       struct dsa_port *link_dp)
 {
 	struct dsa_switch *ds = dp->ds;
 	struct dsa_switch_tree *dst;