mlxsw: spectrum: Set STP state when leaving 802.1D bridge

When a VLAN device leaves a bridge its STP state is set to DISABLED,
which causes the hardware to discard any packets coming through the port
with this VLAN.

Fix that by setting STP state to FORWARDING when the device leaves its
bridge and allow traffic to be directed to CPU.

Fixes: 26f0e7fb

 ("mlxsw: spectrum: Add support for VLAN devices bridging")
Reported-by: Elad Raz <eladr@mellanox.com>
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>