bwn_dma_nextslot
dr->start_transfer(dr, bwn_dma_nextslot(dr, slot));
static int bwn_dma_nextslot(struct bwn_dma_ring *, int);
for (; slot != curslot; slot = bwn_dma_nextslot(dr, slot))
*slot = bwn_dma_nextslot(dr, *slot);
slot = bwn_dma_nextslot(dr, slot);
slot = bwn_dma_nextslot(dr, dr->dr_curslot);