mlxsw: spectrum_router: Pass payload pointer to nexthop update function

Have the caller pass a pointer to the payload of the RATR register to
the function updating a single nexthop / adjacency entry.

In a subsequent patch, this will allow the caller to make sure
replacement was successful by querying the state of the adjacency entry
after replacement and comparing with the initial request.

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