tg3: Consilidate MAC loopback code

The driver puts the device into MAC loopback in two places in the
driver.  This patch consolidates the code into a single routine.

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