ADC channel 0 photodiode detects both infrared + visible light,
but ADC channel 1 just detects infrared. However, the latter is a bit
more sensitive in that range so complete darkness or low light causes
a error condition in which the chan0 - chan1 is negative that
results in a -EAGAIN.
This patch changes the resulting lux1_input sysfs attribute message from
"Resource temporarily unavailable" to a user-grokable lux value of 0.