ixgbe: Do not attempt to perform interrupts in netpoll when down

This patch resolves issues seen when running netconsole and rebooting via
reboot -f.  The issue was due to the fact that we were attempting to
perform interrupt actions when the q_vectors and rings had already been
freed via the ixgbe_shutdown routines.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Mallikarjuna R Chilakala <mallikarjuna.chilakala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>