pinctrl: vt8500: wmt: Fix checking return value of pinctrl_register()

pinctrl_register() returns NULL on error.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Tony Prisk <linux@prisktech.co.nz>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>