leds: trigger: panic: Don't register panic notifier if creating the trigger failed

It doesn't make sense to register the panic notifier if creating the
panic trigger failed.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/8a61e229-5388-46c7-919a-4d18cc7362b2@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>