leds: save the delay values after a successful call to blink_set()

When calling the hardware blinking function implemented by blink_set(),
the delay_on and delay_off values are not preserved across calls.

Fix that and make the "timer" trigger work as expected when hardware
blinking is available.

BEFORE the fix:
  $ cd /sys/class/leds/someled
  $ echo timer > trigger
  $ cat delay_on delay_off
  0
  0
  $ echo 100 > delay_on
  $ cat delay_on delay_off
  0
  0
  $ echo 100 > delay_off
  $ cat delay_on delay_off
  0
  0

AFTER the fix:
  $ cd /sys/class/leds/someled
  $ echo timer > trigger
  $ cat delay_on delay_off
  0
  0
  $ echo 100 > delay_on
  $ cat delay_on delay_off
  100
  0
  $ echo 100 > delay_off
  $ cat delay_on delay_off
  100
  100

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: <stable@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>