mlxsw: spectrum_router: Adjust comments on nexthop fields

The comments assume that nexthops are simple Ethernet nexthops
that are programmed to forward packets to the associated neighbour. This
is no longer the case, as both IPinIP and blackhole nexthops are now
supported.

Adjust the comments to reflect these changes.

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