Flag sensors as invalid on bogus reads

Flag sensors as invalid on bogus reads

Follow-up to the previous commit:

This driver continues to report stale hw.sensors values when reading
them fails, which can easily be observed on a Pinebook Pro after
plugging in the AC cable, causing the hw.sensors.cwfg0.raw0 (battery
remaining minutes) value to jump considerably one or two times before
stalling and becoming incoherent with the rest.

Flag sensors invalid upfront in apm's fashion and mark them OK iff they
yield valid values;   this is what other drivers such as rktemp(4) do,
but the consequence/intention of SENSOR_FINVALID is sysctl(8) and systat(8)
skipping such sensors (until AC gets plugged off again).

OK patrick