leds: trigger: panic: Simplify led_trigger_set_panic

leds: trigger: panic: Simplify led_trigger_set_panic

I don't see why we iterate over all triggers to find the panic trigger.
We *are* the panic trigger. Therefore we also know that the panic
trigger doesn't have an activate() hook. So we can simplify the code
significantly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Link: https://lore.kernel.org/r/84c0fa67-2f03-4474-aa75-914d65d88dd0@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>