mlxsw: spectrum_router: Consolidate mlxsw_sp_nexthop{4, 6}_type_init()

The two functions are now identical, so consolidate them to
mlxsw_sp_nexthop_type_init().

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>