greybus: es2: implement flush callback

Implement the flush callback which is called as part of connection tear
down to flush host-device queues and stop any ongoing transfers.

Note that this should be considered an optimisation of sort since if
the CPort is stuck waiting for credit, the CPort is likely still stuck
when we try to send the cport_shutdown request over it after the
callback returns.

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