Change the ASF heart beat to 5 seconds for faster detection of system
crash. The driver sends the heartbeat every 2 seconds and the ASF
firmware will timeout and reset the device if no heartbeat is received
after 5 seconds. The old scheme of 2 minutes is ineffective.

tg3_write_mem_fast() is added to speed up the IO to send the heartbeat.
When no workaround is needed, it will use direct MMIO to memory space
to write to memory.
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>