[USB] pxa2xx_udc updates, mostly for non-Lubbock hardware

  - IXP 42x UDC support (Greg Weeks)

  - remove Lubbock-specific build assumption (Guennadi Liakhovetski)

  - handle D+ pullup right on iPaqs, e7xx, etc (HH.org)

  - don't unbind() with irqs blocked; matches other controller drivers,
    and network layer expectations

  - handle some deferred ep0 responses better

  - support iso transfers (needs fifo size tracking)