The support for non-dma HCDs is likely the most interesting bit here.

    - makes dma calls behave sensibly when used with host controllers
      that don't use dma (including sl811).  usb_buffer_map() is a nop
      while scatterlist dma mappings fail (as they must).

    - make usb_sg_init() behave sensibly when used with non-dma hcs.
      the urbs are initted with transfer_buffer, not transfer_dma.
      this is the higher level analogue to usb_buffer_map(), so it
      needs to succeed unless there's a Real Error (tm).

    - moves two compatibility inlines from ehci.h into hcd.h so
      it'll be more practical to have the other hcds work in other
      environments (notably lk 2.4) too

    - remove URB_TIMEOUT_KILLED flag ... no device driver tests it;
      hcds don't really (uhci sets it, never reads it; sl811 doesn't
      enable the path that might set it), and it's not well defined.
      if any hcd needs such state, keep it in hc-private storage.

    - in usb_sg_wait(), use yield() instead of schedule() to let
      other activities free resources needed to continue.  (This
      was noted recently by Oliver.)