e1000: fix screaming IRQ

VMWare reports that the e1000 driver has a bug when bringing down the
interface, such that interrupts are not disabled in the hardware but the
driver stops reporting that it consumed the interrupt.

The fix is to set the driver's "down" flag later in the routine,
after all the timers and such have exited, preventing the interrupt
handler from being called and exiting early without handling the
interrupt.

CC: Anupam Chanda <anupamc@vmware.com>
CC: stable kernel <stable@kernel.org>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>