[TG3]: Improve ASF heartbeat.

Change to a different ASF heartbeat message code to improve
reliability.

There were some reports of unintended resets on real time kernels
where the timer may be slow and cause the heartbeat to be late.
Netpoll will also have the same problem because the timer irq will
be unavailable.

Using the new heartbeat code, the ASF firmware will also check the
ring condition before resetting the chip when the heartbeat is
expiring.

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