Merge remote-tracking branch 'kraxel/usb.57' into staging

* kraxel/usb.57:
  ehci: improve expire time calculation
  ehci: implement Interrupt Threshold Control support
  ehci: raise irq in the frame timer
  uhci: initialize expire_time when loading v1 vmstate
  usb: add usb attached scsi emulation
  scsi: add free_request callback