mlxsw: spectrum: Extract mlxsw_sp_stp_spms_state()

Instead of duplicating the decision regarding port forwarding state made
by mlxsw_sp_port_vid_stp_set(), extract the decision-making into a new
function and reuse.

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>