This patch has two small fixes for issues that people
reported to me yesterday:

  - One of the out-of-tree drivers sees odd things
    happening when dma chaining is enabled.  (The
    in-tree drivers seem fine with it.)  So disable
    for now; it's easily enabled if needed.

  - Zero Length Packets (ZLPs):

     * Should now read/write ok with PIO.

     * On DMA endpoints, explicit ZLPs need PIO.
       Until they do, don't allow queuing zero length
       buffers onto DMA endpoints.