mlxsw: spectrum: Don't lose bridge port device during enslavement

Currently, when port netdevs (or their uppers) are enslaved to a bridge,
we simply propagate the CHANGEUPPER event all the way down and lose the
context of the actual netdevice used as the bridge port.

This leads to a lot of information hanging off the ports (and vPorts),
which doesn't logically belong there, such as mrouter indication and
unknown unicast flood state.

Following patches are going to put the mlxsw_sp_port struct on diet and
instead introduce a bridge port struct, where the above mentioned
information belongs. But in order to do that, we need to be able to
determine the bridge port netdevice, so propagate it down.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>