leds: regulator: Add dev helper variable

Instead of repeating the hard to read &pdev->dev just create a
local struct device *dev in probe().

Cc: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>