pinctrl-sx150x: Improve oscio GPIO functions

Move actual code that configures oscio pin into a separate function and
use it instead of calling sx150x_gpio_set to avoid calling
sx150x_pin_is_oscio twice and correctly propagte error code in
sx150x_gpio_direction_output.

Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>