e1000: driver state fixes (race fix)

We were plagued by our interrupt handler posting a watchdog event which
could occur when our adapter was going down in case a late packet arrived
just before e1000_down() finished. This caused the watchdog timer to start
after the NIC was down and keep rescheduling it every N seconds. Once
the driver unloaded it would panic.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>