net: dsa: Fix non static symbol warning

Fixes the following sparse warning:

net/dsa/dsa2.c:680:6: warning:
 symbol '_dsa_unregister_switch' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -677,7 +677,7 @@ int dsa_register_switch(struct dsa_switch *ds, struct device_node *np)
 }
 EXPORT_SYMBOL_GPL(dsa_register_switch);
 
-void _dsa_unregister_switch(struct dsa_switch *ds)
+static void _dsa_unregister_switch(struct dsa_switch *ds)
 {
 	struct dsa_switch_tree *dst = ds->dst;