net: dsa: tag_brcm: legacy: fix untagged rx on unbridged ports for bcm63xx

The internal switch on BCM63XX SoCs will unconditionally add 802.1Q VLAN
tags on egress to CPU when 802.1Q mode is enabled. We do this
unconditionally since commit ed409f3bbaa5 ("net: dsa: b53: Configure
VLANs while not filtering").

This is fine for VLAN aware bridges, but for standalone ports and vlan
unaware bridges this means all packets are tagged with the default VID,
which is 0.

While the kernel will treat that like untagged, this can break userspace
applications processing raw packets, expecting untagged traffic, like
STP daemons.

This also breaks several bridge tests, where the tcpdump output then
does not match the expected output anymore.

Since 0 isn't a valid VID, just strip out the VLAN tag if we encounter
it, unless the priority field is set, since that would be a valid tag
again.

Fixes: 964dbf186eaa ("net: dsa: tag_brcm: add support for legacy tags")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20251027194621.133301-1-jonas.gorski@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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