net: bridge: export also pvid flag in the xstats flags

When I added support to export the vlan entry flags via xstats I forgot to
add support for the pvid since it is manually matched, so check if the
entry matches the vlan_group's pvid and set the flag appropriately.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>