staging: vchiq: Have vchiq_dump_* functions return an error code

These functions currently modify the struct dump_context passed
to them, and set context->actual to -EFAULT in case of error.
The issue is that this is never returned to the user (except
accidentally when things align so that that happens). So, have
these functions return 0 on success and the appropriate error
code otherwise, and return nonzero errors to the user.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcelo Diop-Gonzalez <marcgonzalez@google.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20191120202102.249121-5-marcgonzalez@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>