[iwm] Fix iwm_dma_contig_free(). dma->map is always NULL here.

• When bus_dmamem_alloc is used, the bus_dmamap_t is usually set to NULL, so we were never actually freeing any dma memory allocations done via iwm_dma_contig_alloc(). So we should check dma->vaddr instead of dma->map here.

• Also, the dmamap is actually supposed to be invalidated as part of bus_dmamem_free(), so bus_dmamap_destroy() is never needed here.

Submitted by: Imre Vadasz <imre@vdsz.com>
Approved by: re (gjb)
Obtained from: DragonflyBSD git ef2b29a7ba6ca8a9d2c82ab591c0622227ff84cb