Revert "leds: use led_set_brightness in led_trigger_event"

This reverts commit a0193cbe.

The problem with the original commit was that it caused a warning with
the MMC trigger calling del_timer_sync from hard-irq context.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>
Reported-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>