staging: vchiq: Pass vchiq's message when holding a message

vchi created an opaque structure to be held by services while they
process callback messages. The contents of this opaque structure are the
service handle, which all services already maintain, and a pointer to
vchiq's message structure, struct vchiq_header.

Let's get rid of this opaque structure and directly pass struct
vchiq_header, which is what ultimately vchiq consumes when handling
callback messages.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200629150945.10720-33-nsaenzjulienne@suse.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>