greybus: stop storing dest_cport_id in message

We can derive the destination CPort id of any (outbound) message
from the connection it's operation is associated with.  So we don't
need to store that information in every message.

As a result, we no longer need to record it at message initialization
time.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>