ixgbevf: Handle previously-freed msix_entries

The msix_entries memory can be freed by a previous suspend or
remove, so don't crash on close when it isn't there. Also only
clear the interrupts when the interface is up, because there
aren't any when it is not up.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>