drm/nouveau/mmu: drop unneeded assignment in the nvkm_uvmm_mthd_page()

In order to keep the code style consistency of the whole file,
the 'ret' assignments should be deleted.

The clang_analyzer complains as follows:

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/uvmm.c:317:8:warning:
Although the value storedto 'ret' is used in the enclosing expression,
the value is never actually read from 'ret'.
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Luo penghao <luo.penghao@zte.com.cn>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210821021106.27010-1-luo.penghao@zte.com.cn

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/uvmm.c

@@ -299,7 +299,7 @@ nvkm_uvmm_mthd_page(struct nvkm_uvmm *uvmm, void *argv, u32 argc)
 	page = uvmm->vmm->func->page;
 	for (nr = 0; page[nr].shift; nr++);
 
-	if (!(ret = nvif_unpack(ret, &argv, &argc, args->v0, 0, 0, false))) {
+	if (!(nvif_unpack(ret, &argv, &argc, args->v0, 0, 0, false))) {
 		if ((index = args->v0.index) >= nr)
 			return -EINVAL;
 		type = page[index].type;