vio: Fix allocation sizes

vio: Fix allocation sizes

For both rx and tx, we need an array of bus_dmamap_t and mbuf pointers each.
This results in a size of

  (rxqsize + txqsize) * (sizeof(bus_dmamap_t) + sizeof(struct mbuf *))

The factor 2 before the sizeof(bus_dmamap_t) was too much and we
allocated more than we needed.

OK bluhm@