net: dsa: vsc73xx: add port_stp_state_set function

This isn't a fully functional implementation of 802.1D, but
port_stp_state_set is required for a future tag8021q operations.

This implementation handles properly all states, but vsc73xx doesn't
forward STP packets.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Link: https://patch.msgid.link/20240713211620.1125910-2-paweldembicki@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>