xhci: Clear EHB bit only at end of interrupt handler

xhci: Clear EHB bit only at end of interrupt handler

The Event Handler Busy bit shall be cleared by software when the Event
Ring is empty.  The xHC is thereby informed that it may raise another
interrupt once it has enqueued new events (sec 4.17.2).

However since commit dc0ffbea5729 ("usb: host: xhci: update event ring
dequeue pointer on purpose"), the EHB bit is already cleared after half
a segment has been processed.

As a result, spurious interrupts may occur:

- xhci_irq() processes half a segment, clears EHB, continues processing
  remaining events.
- xHC enqueues new events.  Because EHB has been cleared, xHC sets
  Interrupt Pending bit.  Interrupt moderation countdown begins.
- Meanwhile xhci_irq() continues processing events.  Interrupt
  moderation countdown reaches zero, so an MSI interrupt is signaled.
- xhci_irq() empties the Event Ring, clears EHB again and is done.
- Because an MSI interrupt has been signaled, xhci_irq() is run again.
  It discovers there's nothing to do and returns IRQ_NONE.

Avoid by clearing the EHB bit only at the end of xhci_irq().

Fixes: dc0ffbea5729 ("usb: host: xhci: update event ring dequeue pointer on purpose")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org # v5.5+
Cc: Peter Chen <peter.chen@kernel.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20230915143108.1532163-4-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>