drm/nouveau/device: fix changing endianess code to work on older GPUs

With this we try to detect if the endianess switch works and assume LE if
not. Suggested by Ben.

Fixes: 51c05340

 ("drm/nouveau/device: detect if changing endianness failed")
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Cc: <stable@vger.kernel.org> # v5.8+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>