greybus: audio: Split start and stop APBridgeA requests

Provide finer-grained control of the audio streaming on APB1 by
splitting the transmit/receive start and stop requests into prepare,
start, stop, and shutdown.

CC: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>