tg3: Futureproof the loopback test

There are other multiqueue modes 5717 and 5719 devices can assume.  This
patch makes sure that the loopback test is safe, should those other
modes be enabled in the future.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>