sfxge(4): allow to have no NIC handle on Rx datapath in DPDK PMD

It is required to minimize RxQ context in the driver or avoid chaising
for the NIC handle in adapter (global per-interface) structure.

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D8950