led: core: Fix brightness setting when setting delay_off=0

With the current code, the following sequence won't work :
echo timer > trigger

echo 0 >  delay_off
* at this point we call
** led_delay_off_store
** led_blink_set
*** stop timer
** led_blink_setup
** led_set_software_blink
*** if !delay_on, led off
*** if !delay_off, set led_set_brightness_nosleep <--- LED_BLINK_SW is set but timer is stop
*** otherwise start timer/set LED_BLINK_SW flag

echo xxx > brightness
* led_set_brightness
** if LED_BLINK_SW
*** if brightness=0, led off
*** else apply brightness if next timer <--- timer is stop, and will never apply new setting
** otherwise set led_set_brightness_nosleep

To fix that, when we delete the timer, we should clear LED_BLINK_SW.

Cc: linux-leds@vger.kernel.org
Signed-off-by: Matthieu CASTET <matthieu.castet@parrot.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>