greybus: operation: add support for incoming unidirectional operations

Add support for incoming, unidirectional operations where the sender of
a request does not care about a response.

Unidirectional operations have an operation id of 0.

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