mlxsw: spectrum_router: Remove RTNL assertion

Remove the RTNL assertion in the nexthop notifier block. The assertion
is not needed given RTNL is never assumed to be taken.

This is a preparation for future patches where mlxsw will start handling
nexthop events that are not always sent with RTNL held.

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