tg3: Turn off ASF "driver alive" heartbeats for APE

The ENABLE_ASF flag is set when DASH is enabled on the NIC, but DASH
does not run on the RX CPU.  Instead it runs on the APE.
Consequently, the driver does not need to send "driver alive" updates
to the RX CPU when the APE is present.

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>