doc: convert ixgbe guide to new offload API
doc: convert ixgbe guide to new offload API

Cleanup document to convert old rxmode flags to new offload
bit mask.

Signed-off-by: Qi Zhang <9e9e58ffa71a29bb7b87766b362515be648fcbe0@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk