leds: tca6507: cosmetic change: use helper variable

Use helper variable dev instead of always writing &client->dev.

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