team: fall back to hash if table entry is empty

If the hash to port mapping table does not have a valid port (i.e. when
a port goes down), fall back to the simple hashing mechanism to avoid
dropping packets.

Signed-off-by: Jim Hanko <hanko@drivescale.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>