staging: vchiq: Move message queue into struct vchiq_service

staging: vchiq: Move message queue into struct vchiq_service

This has historically been handled by vchi, but there is no reason why
this couldn't be handled directly in vchiq.

The patch tries to avoid altering any behavior, with the exception of
the msg_queue size, which is now fixed to VCHIQ_MAX_SLOTS (it was set to
VCHIQ_MAX_SLOTS / 2). This is done to match vchiq's user_service message
queue, which could be merged with this one in the future.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200629150945.10720-24-nsaenzjulienne@suse.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>