tg3: Organize loopback test failure flags

As more test modes are added to each loopback mode, the need to
organise the results increases.  This patch groups the results by
loopback mode, and then by test mode.

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