drm/nouveau/tegra: Fix error pointer vs NULL return in nvkm_device_tegra_resource_addr()

The nvkm_device_tegra_resource() function returns a mix of error pointers
and NULL.  The callers only expect it to return NULL on error.  Change it
to only return NULL.

Fixes: 76b8f81a ("drm/nouveau: improve handling of 64-bit BARs")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://lore.kernel.org/dri-devel/334404bdf60765cb5a8e855a74c688bc537531ee.camel@nvidia.com/T/#t