Pass the correct dma tag to the load buffer function. Passing t only

Pass the correct dma tag to the load buffer function.  Passing t only
passes the tag where the pointer to the simplebus_dmamap_load_buffer()
wrapper is stored in.  We want to call the load buffer function that
was passed from our parent device, so we have to pass the parent's tag.
Otherwise we end up in a recursive and neverending loop.

This only occurs when a simplebus attaches to a simplebus, which we
apparently didn't have so far.