tg3: Turn off the debug UART for 57765

If the debug UART is left enabled, the LEDs will not work properly.
This patch disables the debug UART.

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>