gpio: mmio: replace open-coded for_each_set_bit()

Use for_each_set_bit() instead of open-coding it to simplify the code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200713154429.23662-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>