pinctrl: baytrail: Use defined macro instead of magic in byt_get_gpio_mux()

pinctrl: baytrail: Use defined macro instead of magic in byt_get_gpio_mux()

By the fact byt_get_gpio_mux() returns a value of mux settings as
it is represented in hardware. Use defined macro instead of magic numbers
to clarify this.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20190703003018.75186-1-andriy.shevchenko@linux.intel.com
Reviewed-By: Enrico Weigelt <info@metux.net>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>