iflib: netmap: improve rxsync to support IFLIB_HAS_RXCQ

For drivers with IFLIB_HAS_RXCQ set, there is a separate completion
queue. In this case, the netmap rxsync routine needs to update
rxq->ifr_cq_cidx in the same way it is updated by iflib_rxeof().
This improves the situation for vmx(4) and bnxt(4) drivers, which
use iflib and have the IFLIB_HAS_RXCQ bit set.

PR: 248494
MFC after: 3 weeks