mlxsw: Convert RIF-has-netdevice queries to a dedicated helper

In a number of places, a netdevice underlying a RIF is obtained only to
check if it a NULL pointer. In order to clean up the interface between the
router and the other modules, add a new helper to specifically answer this
question, and convert the relevant uses to this new interface.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>