mlxsw: spectrum_router: Only provide MAC address for valid nexthops

The helper returns the MAC address associated with the nexthop. It is
only valid when the nexthop forwards packets and when it is an Ethernet
nexthop. Reflect this in the checks the helper is performing.

This is not an issue because the sole caller of the function only
invokes it for such nexthops.

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