gpiolib: Use proper type for bias enumerator in gpio_set_bias()

First of all, bias has a special type as being a part of enum pin_config_param.
Second, 0 is also defined bias which is equivalent to BUS_HOLD.

Taking into account above, change type of bias variable and refactor
gpio_set_bias() in a way that it doesn't use BUS_HOLD as a place holder.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20201009184359.16427-1-andriy.shevchenko@linux.intel.com


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