drm/nouveau/clk/gk20a: fix VCO bit mask

Fix the mask specified to switch to VCO mode was given as an (incorrect)
immediate value. Although the side-effect happens to be the same, this
is clearly incorrect.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>