leds: ns2: cosmetic variable rename

Rename variable led_dat to led in various functions.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Simon Guinot <simon.guinot@sequanux.org>
Cc: Simon Guinot <sguinot@lacie.com>
Cc: Vincent Donnefort <vdonnefort@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>