leds: Improve and export led_update_brightness

led_update_brightness helper function used to be exploited only locally
in the led-class.c module, where its result was being passed to the
brightness_show sysfs callback. With the introduction of v4l2-flash
subdevice the same functionality becomes required for reading current
brightness from a LED device. This patch adds checking of return value
of the brightness_get callback and moves the led_update_brightness()
function to the LED subsystem public API.

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>