mlxsw: spectrum_router: Encapsulate nexthop update in a function

Encapsulate this functionality in a separate function, so that it could
be invoked by follow-up patches, when replacing a nexthop bucket that is
part of a resilient nexthop group.

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