pinctrl: stm32: fix compile error and modernize

- Fix the dev->parent assignment compile error
- Use gpiochip_get_data() to get the data pointer for the
  banks

Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>