leds: leds-pwm: Set led_classdev max_brightness

Currently the driver leds-pwm doesn't set max_brightness for the led device
although it's platform data proides a maximum brightness. Instead it stores its
own private driver struct. The max_brightness defaults to 255 for led device if
it has not been set.
As a result any leds-pwm device with a different maximum brightness will show
incorrect behavior, as it is posible to either set a longer then period duty
time or not be able to switch the led to full brightness.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>