mlxsw: spectrum_switchdev: Publish two functions

Publish the existing function mlxsw_sp_bridge_port_find(), and add
another service accessor mlxsw_sp_bridge_port_stp_state(). Publish both
in a new file spectrum_switchdev.h.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>