bridge: simplify the stp_state_store by calling store_bridge_parm

There are some repetitive codes in stp_state_store, we can remove
them by calling store_bridge_parm.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>