staging: vchi: Rework vchi_msg_hold() to match vchiq_msg_hold()

The services have access to struct vchiq_header's internals, so we can
let them get the data pointer. This pretty much makes both functions
exactly the same, which will allow us to make a switch to
vchiq_msg_hold() further down the road.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200629150945.10720-34-nsaenzjulienne@suse.de


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