pinctrl: stm32: Fix bad function call

In stm32_pconf_parse_conf function, stm32_pmx_gpio_set_direction is
called with wrong parameter value. Indeed, using NULL value for range
will raise an oops.

Fixes: aceb16dc ("pinctrl: Add STM32 MCUs support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>