[TG3]: Limit 5784 / 5764 to MAC LED mode

Most 5784 / 5764 LED modes do not work as expected because of a hardware
bug.  This patch forces the LED mode to be in MAC LED mode.

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