greybus: operation: add support for initiating unidirectional operations

Add support for initiating unidirectional operations, that is, sending
requests that do not require responses.

Note that we already handle incoming unidirectional operations.

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