mlxsw: spectrum: Reword an error message for Q-in-Q veto

'Uppers' is not clear enough for all users when referring to upper
devices.

Reword the error message so it will be clearer.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>