gpio: 74xx-mmio: use bits.h macros for all masks

Make use of the GENMASK() (far less error-prone, far more concise).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>