drm/nouveau: fix reclocking on nv40

In commit 77145f1c was introduced
error which cause that reclocking on nv40 not working anymore.
There is missing assigment of return value from pll_calc to ret.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>