ixgbe: 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.

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>