gpio: max732x: Fix I2C dummy device resource leak on probe failure

In max732x_probe() driver allocates dummy I2C device (if number of ports
is greater than 8) however it is not unregistered if probe fails later.
Fix the leak by unregistering dummy I2C device if it was allocated.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>