e1000: Avoid unhandled IRQ

If hardware asserted an interrupt and driver is down,
then there is nothing to do so return IRQ_HANDLED
instead of IRQ_NONE. Returning IRQ_NONE in above
situation causes screaming IRQ on virtual machines.

CC: Andy Gospodarek <gospo@redhat.com>
Signed-off-by: Tushar Dave <tushar.n.dave@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by:  <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>