leds: avoid using DEVICE_ATTR macro for max_brightness attribute

leds: avoid using DEVICE_ATTR macro for max_brightness attribute

Make definition of the brightness related sysfs attributes
consistent. The modification entails change of the function
name: led_max_brightness_show -> max_brightness_show

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