i8k: Fix temperature bug handling in i8k_get_temp()
Static array prev[] was incorrectly initialized. It should be initialized to
some "invalid" temperature value (above I8K_MAX_TEMP).
Next, function should store "invalid" value to prev[] (above I8K_MAX_TEMP),
not valid (= I8K_MAX_TEMP) because whole temperature bug handling will not
work.
And last part, to not break existing detection of temperature sensors, register
them also if i8k report too high temperature (above I8K_MAX_TEMP). This is
needed because some sensors are sometimes turned off (e.g sensor on GPU which
can be turned off/on) and in this case SMM report too high value.
To prevent reporting "invalid" values to userspace, return -EINVAL. In this case
sensors which are currently turned off (e.g optimus/powerexpress/enduro gpu)
are reported as "N/A" by lm-sensors package.