mlxsw: spectrum: Store pointer to default port VLAN in port struct

Subsequent patches will need to access the default port VLAN. Since this
VLAN will exist throughout the lifetime of the port, simply store it in
the port's struct.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>