mlxsw: spectrum_router: Access rif->dev from params in mlxsw_sp_rif_create()

The previous patch added a helper to access a netdevice given a RIF. Using
this helper in mlxsw_sp_rif_create() is unreasonable: the netdevice was
given in RIF creation parameters. Just take it there.

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