ixgbe: reduce checker warnings

Fix the sparse warnings in the ixgbe crypto offload code. These
changes were made in the most conservative way (force cast)
in order to hopefully not break the code. I suspect that the
code might still be broken on big-endian architectures, but
no one is complaining, so I'm just leaving it functionally
the same.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: Shannon Nelson <snelson@pensando.io>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>