nv30: Fix parsing of perf table

Perf tables v 1.2 and 1.3 (seen on Geforce FX/ 5) are not long enough
to store the voltage label/id

v2 - Remove comment from the code

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>