i40e, xsk: uninitialized variable in i40e_clean_rx_irq_zc()

The "failure" variable is used without being initialized.  It should be
set to false.

Fixes: 8cbf7414

 ("i40e, xsk: move buffer allocation out of the Rx processing loop")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Tested-by: George Kuruvinakunnel <george.kuruvinakunnel@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>