leds: tca6507: use fwnode API instead of OF

Convert to use fwnode API instead of OF. It is more generic and if
someone wants to use this driver without device-tree yet still, they
will be able to via swnode fwnodes.

Remove the gpio setup function from platdata.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: NeilBrown <neilb@suse.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>