greybus: esX: use one byte to encode cport ids in header

We now limit the maximum value for both host and module CPort ids,
and we know they can always be represented in a single byte.

Make use of this by using only one of the two pad bytes for encoding
the CPort id in a message header.

(Note that we have never used a CPort higher than 255.  Encoding
such a small CPort id in little endian 2-byte format has the same
result as what is done here.)

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