gpu, drm, sis: Don't return uninitialized variable from sis_driver_load()

In sis_driver_load(), the only use of 'ret' is as the return value
from the function, unfortunately it is never initialized, so the
function just returns garbage when it succeeds.
To fix that, remove the variable and just return 0 directly on success.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>