mlxsw: spectrum_router: Access nhgi->rif through a helper

To abstract away deduction of RIF from the corresponding next hop group
info (NHGI), mlxsw currently uses a macro. In its current form, that macro
is impossible to extend to more general computation. Therefore introduce a
helper, mlxsw_sp_nhgi_rif(), and use it throughout. This will make it
possible to change the deduction path easily later on.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>