Fix DMA synchronization in the ENA driver Tx and Rx paths

The DMA in FreeBSD requires explicit synchronization. ENA driver was
only doing PREREAD and PREWRITE synchronizations. Missing
bus_dmamap_sync() calls were added.

It is also required to synchronize DMA engine before unloading DMA map.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.