drivers: led: lp5562: reuse engine already linked

Right now "lp5562_led_blink" unconditionally find and assign a engine
that is not executing at the moment. This is an issue, if 3 "led" are
already using an engine. That is a call to "lp5562_led_blink" could
terminate with error, even if the "led" is already using an engine.
This commit fixes this issue by adding a check to see if engine is
linked and reuse it, otherwise assign a free engine.

Signed-off-by: Anuj Pathak <anuj@croxel.com>