leds: leds-lp50xx: Handle reg to get correct multi_index

mc_subled used for multi_index needs well defined array indexes,
to guarantee the desired result, use reg for that.

If devicetree child nodes is processed in random or reverse order
you may end up with multi_index "blue green red" instead of the expected
"red green blue".
If user space apps uses multi_index to deduce how to control the leds
they would most likely be broken without this patch if devicetree
processing is reversed (which it appears to be).

arch/arm/boot/dts/aspeed/aspeed-bmc-facebook-fuji.dts has reg set
but I don't see how it can have worked without this change.

If reg is not set, an error is returned,
If reg is out of range, an error is returned.
reg within led child nodes starts with 0, to map to the iout in each bank.

Signed-off-by: Johan Adolfsson <johan.adolfsson@axis.com>
Reviewed-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Link: https://lore.kernel.org/r/20250617-led-fix-v7-1-cdbe8efc88fa@axis.com


Signed-off-by: Lee Jones <lee@kernel.org>