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: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>