drm/nouveau/nvif: remove const attribute from nvif_mclass

Similar to commit 0bf8bf50 ("module: Remove
const attribute from alias for MODULE_DEVICE_TABLE")

Fixes many -Wduplicate-decl-specifier warnings due to the combination of
const typeof() of already const variables.
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/include/nvif/object.h

@@ -78,7 +78,7 @@ struct nvif_mclass {
 #define nvif_mclass(o,m) ({                                                    \
 	struct nvif_object *object = (o);                                      \
 	struct nvif_sclass *sclass;                                            \
-	const typeof(m[0]) *mclass = (m);                                      \
+	typeof(m[0]) *mclass = (m);                                            \
 	int ret = -ENODEV;                                                     \
 	int cnt, i, j;                                                         \
                                                                                \