i2c/tsl2550: Fix lux value in dark environment

I've tested TSL2550 driver and I've found a bug: when light is off, 
returned value from tsl2550_calculate_lux function is -1 when it should 
be 0 (sensor correctly read that light was off).

I think the bug is that a zero c0 value (approximated value of ch0) is 
misinterpreted as an error. 

Signed-off-by: Michele Jr De Candia <michele.decandia@valueteam.com>
Acked-by: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: Jean Delvare <khali@linux-fr.org>