pinctrl: check ops->pin_config_set in pinconf_set_config()

pinctrl: check ops->pin_config_set in pinconf_set_config()

pinconf_set_config() is called by pinctrl_gpio_set_config().
If a GPIO driver is backed by a pinctrl driver and it does not
support .pin_config_set() hook, it causes NULL pointer dereference.

Fixes: 15381bc7c7f5 ("pinctrl: Allow configuration of pins from gpiolib based drivers")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>