tg3: Disable EEE during loopback tests

EEE interferes with the hardware's ability to loop a packet back to the
host.  This patch disables the feature for the duration of the test.

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