Define a new function operation_cancel() that cancels an
outstanding operation. Use it to clear out any operations that
might be pending at the time a connection is torn down.
Note: This code isn't really functional yet, partially because
greybus_kill_gbuf() is not implemented.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>