gpio: msic: Fix calculating register address in msic_gpio_to_oreg()

In the case offset is 20 ... 23, the equation to get the register should be:
        INTEL_MSIC_GPIO1HV0CTLO - offset + 20

With above equation, we can get below mapping between offset and the register:
        offset is 20: INTEL_MSIC_GPIO1HV0CTLO
        offset is 21: INTEL_MSIC_GPIO1HV1CTLO
        offset is 22: INTEL_MSIC_GPIO1HV2CTLO
        offset is 23: INTEL_MSIC_GPIO1HV3CTLO

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>