ohci: clear pending SOF on suspend

On overcommitted CPU, kernel can be so slow that an interrupt can
be triggered by the device whereas the driver is not ready to receive
it. This drives us into an infinite loop.

On suspend, if a SOF interrupt is raised between the stop of the
device processing and the change of the device internal state to
OHCI_USB_SUSPEND (QEMU stops SOF timer on this state change), this
interrupt is never acknowledged.

This patch clears pending SOF interrupt on OHCI_USB_SUSPEND setting.

Some details:

- ohci_irq(): the OHCI interrupt handler, acknowledges the SOF IRQ
  only if the state of the driver (rh_state) is OHCI_STATE_RUNNING.
  So if this interrupt happens and the driver is not in this state,
  the function is called again and again, moving the system to a
  CPU starvation.

- ohci_rh_suspend(): the function stop the operation and acknowledge
  pending interrupts (but doesn't disable it). Later in the function,
  the device is moved to OHCI_SUSPEND_STATE, and the driver to
  OHCI_RH_SUSPENDED. If between the moment when the interrupt is
  acknowledged and the moment when the device is suspended a new
  interrupt is raised, it will be never acknowledged because the
  driver is now not in OHCI_RH_RUNNING state.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 1452109525-32150-3-git-send-email-lvivier@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
