greybus: operation: fix cancellation use-after-free

The final reference of an operation will be put after its completion
handler has run, so we must not drop the reference if it has already
been scheduled to avoid use-after-free.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>