realtek: rtl93xx: Trap BPDU management frames

realtek: rtl93xx: Trap BPDU management frames

BPDU frames like STP must be processed by each switch (bridge) which
supports STP. It must not be forwarded to avoid confusing the STP state of
other STP participants. It is essential to be an active participant of STP.
The software bridge automatically takes care of forwarding the BPDUs to
other ports when STP is disabled and the hardware switch should not
interfere.

Signed-off-by: Harshal Gohel <hg@simonwunderlich.de>
Signed-off-by: Sven Eckelmann <se@simonwunderlich.de>
Link: https://github.com/openwrt/openwrt/pull/20414
Signed-off-by: Robert Marko <robimarko@gmail.com>