ixgbe: fix broken ipsec Rx with proper cast on spi

Fix up a cast problem introduced by a sparse cleanup patch.  This fixes
a problem where the encrypted packets were not recognized on Rx and
subsequently dropped.

Fixes: 9cfbfa70 ("ixgbe: cleanup sparse warnings")
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>