wifi: iwlegacy: Add null pointer check to il_leds_init()

kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure.

Cc: Kunwu Chan <kunwu.chan@hotmail.com>
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231211033019.238149-1-chentao@kylinos.cn