greybus: esX: encapsulate packing cport id into header

For the ES1 and ES2 host interfaces we encode the CPort ID over
which the message should be sent within the message itself.  The
CPort ID is recorded in unused pad bytes found in the operation
message header in order to avoid introducing misaligned messages.

This patch defines some helper routines to abstract this activity.

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