i40e: Ignore zero length packets
i40e: Ignore zero length packets

Buffers that claim to have zero length packets aren't ready yet.
See 0e626ff7ccbfc and d57c0e08c7016 in linux.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap