tg3: Bypass power source switching for 57765

The 57765 repurposes all the GPIOs normally used to switch power sources
when configured as a NIC device.  This patch changes the code to avoid
touching the GPIOs for this asic rev.

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>