leds: use led_set_brightness in led_trigger_event

Fix led_trigger_event() to use led_set_brightness() instead of
__led_set_brightness(), so that any pending blink timer is stopped before
setting the new brightness value.  Without this fix LED status may be
overridden by a pending timer.

This allows a trigger to use a mix of led_trigger_event(),
led_trigger_blink() and led_trigger_blink_oneshot() without races.

(applied over: leds: Rename led_brightness_set() to led_set_brightness())

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>
Cc: Shuah Khan <shuahkhan@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>