leds: tlc591xx: update the maximum brightness

The TLC chips actually offer 257 levels:
- 0: led OFF
- 1-255: Led dimmed is using a PWM. The duty cycle range from 0.4% to 99.6%
- 256: led fully ON

Fixes: e370d010 ("leds: tlc591xx: Driver for the TI 8/16 Channel i2c LED driver")
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>