mlxsw: spectrum: Sanitize VLAN interface's uppers

Currently, only VRF and macvlan uppers are supported on top of VLAN
device configured over a bridge, so make sure the driver forbids other
uppers.

Note that enslavement to a VRF is handled earlier in the notification
block, so there is no need to check for a VRF upper here.

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