gpiolib: Use GPIOD_OUT_{LOW,HIGH} macros in open drain ones

There should not be anything more than stated by the name of newly
introduced constants, i.e.
	GPIOD_OUT_LOW_OPEN_DRAIN == GPIOD_OUT_LOW + open drain
and nothing more.

Make it better to read and slightly more robust by using GPIOD_OUT_LOW
and GPIOD_OUT_HIGH constants with open drain flag.

No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>