xhci: Clear stopped_td when Stop Endpoint command completes.

When an URB is cancelled, the xHCI driver issues a Stop Endpoint command
so that it can manipulate the ring and remove the transfer.  The xHC
hardware then places a transfer event with the completion code "Stopped"
or "Stopped Invalid" to let the driver know what TD it was in the middle
of processing.  This TD and TRB is stored in ep->stopped_td and
ep->stopped_trb.  These pointers are also used in handling stalled
endpoints.

By design, the Stop Endpoint command can race with URB completion.  By
the time the Stop Endpoint command is handled, the URBs to be cancelled
may have been given back to the driver.  Unfortunately, the stopped_td
and stopped_trb pointers were not getting cleared in this case.

The USB core unconditionally tries to reset the toggle bits on any
endpoints when a new alternate interface setting is installed.  When the
xHCI driver saw that ep->stopped_td was still set from the Stop Endpoint
command, xhci_reset_endpoint assumed the endpoint was actually stalled,
and attempted to clean up the endpoint rings.  This would manifest
itself in a failed Reset Endpoint command and failed Set TR dequeue
Pointer command after a successful Configure Endpoint command.  It may
have also been causing driver oops when the stopped_td was accessed.

This patch should be backported to stable kernels since 2.6.31.  Before
2.6.33, stopped_td was found in the xhci_endpoint_ring, not the
xhci_virt_ep.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>