drm/nouveau: simplify the return expression of nouveau_debugfs_init()

Simplify the return expression.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Reviewed-by: Lyude Paul <lyude@redhat.com>
[fixed an indenting error before pushing]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429090309.3853003-1-chi.minghao@zte.com.cn