* call netif_tx_disable after netif_poll_disable, fixing TX race,
  in tg3_netif_stop
* follow the ordering of the tg3_netif_stop change, and enable
  poll after waking TX, in tg3_netif_start
* after doing those two steps in tg3_netif_start, check for work
  using new helper function tg3_cond_int
* add helper function tg3_cond_int, which delivers an interrupt
  if and only if the status block was updated (i.e. if work
  is likely to be available)