i2c/tsl2550: Fix lux value in extended mode

According to the TAOS Application Note 'Controlling a Backlight with
the TSL2550 Ambient Light Sensor' (page 14), the actual lux value in
extended mode should be obtained multiplying the calculated lux value
by 5.

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