drm/nouveau/acr: fix a couple NULL vs IS_ERR() checks

The nvkm_acr_lsfw_add() function never returns NULL.  It returns error
pointers on error.

Fixes: 22dcda45

 ("drm/nouveau/acr: implement new subdev to replace "secure boot"")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211118111314.GB1147@kili