greybus: introduce cport_id_valid()

Define a public predicate that defines whether a CPort ID is valid.

Use it in the message_send() routine, and make the message reported
more accurately reflect the error.  Also use it to check whether the
CPort ID in a received message is valid; if it is not, just drop the
message.

Get rid of local variable "buffer" in message_send(); it adds no
value.

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