If we leave max_value at the default value of zero led_pwm_set above
will divide by zero in do_div. The binding makes max-brightness
a mandatory property, so have the driver treat it as such as well.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>