leds: make brightness type consistent across whole subsystem

Documentations states that brightness units type is enum led_brightness
and this is the type used by the led API functions. Adjust the type
of brightness variables in the struct led_classdev accordingly.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Bryan Wu <cooloney@gmail.com>