drm/nouveau/bios/iccsense: rails for power sensors have a mask of 0xf8 for version 0x10

I only saw those values inside the vbios: 0xff, 0xfd, 0xfc, 0xfa for valid
rails.

No idea what the lower value does, but at least we get power readings on
a lot of Fermi GPUs with that.

v2: add missing parentheses

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>