mlxsw: spectrum: Update a comment

The position of netdevice notifier registration no longer depends on the
router initialization, because the event handler no longer dispatches to
the router code. Update the comment at the registration to that effect.

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