tg3: Eliminate timer race with reset_task

During shutdown, it is impossible to reliably disable the timer and
reset_task threads.  Each thread can schedule the other, which leads to
shutdown code that chases its tail.

To fix the problem, this patch removes the ability of tg3_reset_task to
schedule a new timer thread.  To support this change, tg3_timer no
longer terminates itself, but rather goes into a polling mode.

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