Check handling around `ACK_CCI` command with just a connector status

[This code](https://github.com/OpenDevicePartnership/embedded-usb-pd/blob/df9cecdf66b6399bc75b7f985c06b443fa80b82d/src/ucsi/ppm/state_machine.rs#L120) would return the ACK_CCI command to execute if the command complete flag isn't set. However, I'm not actually sure if the spec supports this usage or if every connector change ack has to be combined with a command complete ack.