greybus: refactor gb_connection_recv()

Define two helper functions to break down handling of a received
message.  One is used to handle receiving an incoming request
message, the other for a response message.

Three other changes are made:
    - We verify message size recorded in the message header does not
      exceed the amount of data that's arriving.
    - We no longer warn if a request' recorded message size differs
      from the number of bytes that have arrived.
    - We now record the operation id for an incoming request.

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