watchdog: wm831x_wdt: use devm_gpio_request_one()

Use devm_gpio_request_one() to make cleanup paths simpler.
Also, GPIOF_DIR_OUT | GPIOF_INIT_LOW is replaced with
GPIOF_OUT_INIT_LOW.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>