tg3: Prevent spurious tx timeout by setting carrier off before tx disable.

The watchdog will not trigger when the carrier is off when reconfiguring
the device.  Because carrier state is now off during reset, we need to
introduce a link_up flag to keep track of link state during PHY setup.

Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>