greybus: send operation response messages

Define a helper function gb_operation_response_alloc() and use it
to allocate the response buffer for outgoing operations in
gb_operation_create_common(.

Use it also in  gb_operation_response_send() if the caller has not
allocated a response buffer.

Once a response buffer is allocated, fill in its result code and
send it.

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