leds: Don't allow brightness values greater than max_brightness

Since commit 4d71a4a1 ("leds: Add support for setting brightness in
a synchronous way") the value passed to brightness_set() is no longer
limited to max_brightness and can be different from the internally saved
brightness value.

Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>