greybus: operation: fix another cancellation use-after-free

An incoming operation could already be scheduled even if
gb_operation_result_set succeeds as its initial status is -EINPROGRESS.

Avoid potential use-after-free by never dropping the reference count for
incoming operations as part of cancellation.

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