greybus: operation: add unidirectional-operation flag

Add flag to identify unidirectional operations.

Use convenience helper rather than open coding the identification when
suppressing response messages.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>