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