pinctrl: mediatek: correct error handling

If no GPIO controller is found, the return value should not depend on a
random value on the stack. Initialize variable ret.

The problem was indicated by cppcheck.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>