ixgb: check down state before enable irq

in order to prevent the case where poll_disable is waiting
on our device to permanently, check the flag to make sure we're not
down or closing down before re-enabling interrupts.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>