leds: unify the location of led-trigger API

Part of led-trigger API was in the private drivers/leds/leds.h header.
Move it to the include/linux/leds.h header to unify the API location
and announce it as public. It has been already exported from
led-triggers.c with EXPORT_SYMBOL_GPL macro. The no-op definitions are
changed from macros to inline to match the style of the surrounding code.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>