ixgbevf: Fix panics in the VF driver

Fix panics in the VF driver that occur when you bring it down after
having already brought the PF down.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>