gpiolib: Make use of assign_bit() API

We have for some time the assign_bit() API to replace open coded

	if (foo)
		set_bit(n, bar);
	else
		clear_bit(n, bar);

Use this API in GPIO library code.

No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20191204194229.64251-2-andriy.shevchenko@linux.intel.com


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