greybus: cancel operation on timeout

If an operation times out, we need to cancel whatever message it
has in-flight.  Do that instead of completing the operation, in the
timeout handler.  When the in-flight request message is canceled its
completion function will lead to the proper completion of the
operation.

Change gb_operation_cancel() so it takes the errno that it's
supposed to assign as the result of the operation.

Note that we want to preserve the original -ETIMEDOUT error, so
don't overwrite the operation result value if it has already been
set.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>