tg3: Disable MAC loopback test for CPMU devices

On CPMU devices, the MAC loopback test does not test any important paths
the phy loopback test doesn't also test.  The phy loopback test is the
more comprehensive test.  This patch disables the MAC loopback test for
these devices.

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