leds: lm355x: fix attribute-creation race

Use the attribute groups of the led-class to create the indicator-LED
attributes during probe in order to avoid racing with userspace.

Note that the comment about the pattern attribute only being for LM3554
was incorrect and did not match the code (the original leds-lm3556
driver had the attribute before LM3554 support was added).

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Bryan Wu <cooloney@gmail.com>