Fix teardown of static DMA allocations in various NIC drivers:
- Add missing calls to bus_dmamap_unload() in et(4).
- Check the bus address against 0 to decide when to call
  bus_dmamap_unload() instead of comparing the bus_dma map against NULL.
- Check the virtual address against NULL to decide when to call
  bus_dmamem_free() instead of comparing the bus_dma map against NULL.
- Don't clear bus_dma map pointers to NULL for static allocations.
  Instead, treat the value as completely opaque.
- Pass the correct virtual address to bus_dmamem_free() in wpi(4) instead
  of trying to free a pointer to the virtual address.

Reviewed by:	yongari