drm/nouveau/volt/gf117: fix speedo readout register

GF117 appears to use the same register as GK104 (but still with the
general Fermi readout mechanism).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108980


Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>