greybus: connection: add flush host-device callback

Add a new host-device callback to flush any host-device queues,
including any ongoing transfers, as part of connection tear down.

The host-device buffers are flushed after the disconnecting operation
have completed and the AP queues have been emptied.

This can be used to flush already queued messages which upon reception
would have been discarded by the remote end anyway.

Note that this does not remove the need to flush any host-device queues
as part of CPort disable which needs to make the CPort ready for reuse
after deallocating all associated state and resetting the port.

Suggested-by: Mitchell Tasman <tasman@leaflabs.com>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>