net: dsa: pass bridge device when a port leaves

Upon reception of the NETDEV_CHANGEUPPER, a leaving port is already
unbridged, so reflect this by assigning the port's bridge_dev pointer to
NULL before calling the port_bridge_leave DSA driver operation.

Now that the bridge_dev pointer is exposed to the drivers, reflecting
the current state of the DSA switch fabric is necessary for the drivers
to adjust their port based VLANs correctly.

Pass the bridge device pointer to the port_bridge_leave operation so
that drivers have all information to re-program their chips properly,
and do not need to cache it anymore.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>