staging: vchi: Get rid of vchiq_shim's message callback

staging: vchi: Get rid of vchiq_shim's message callback

As vchiq_shim's callback does nothing aside from pushing messages into
the service's queue, let's bypass it and jump directly to the service's
callbacks, letting them choose whether to use the message queue.

It turns out most services don't need to use the message queue, which
makes for simpler code in the end.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200629150945.10720-29-nsaenzjulienne@suse.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>