xhci: flush endpoint start to reduce race risk with stop endpoint command.

Stop endpoint command fails with "context state error" if the endpoint is
already stopped.

This case was observed when a previous URB cancel had just completed and
rang the doorbell to restart the ring, when a new URB cancel queued a stop
endpoint command.
>From xHC hardware pov the endpoint had not yet started, so the stop
endpoint command failed with context state error.
Right after this the doorbell ring took effect and ring was restarted.

Interrupt handler saw a stop endpoint command completion event with
"context state error" and discovered that the ring was back up in
running state.

flushing the write reduces these cases in stress testing, but does not
completely remove the issue.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20210129130044.206855-15-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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