pinctrl: stm32: Declare stm32_pmx_get_mode() as static

pinctrl: stm32: Declare stm32_pmx_get_mode() as static

The commit acaa037970f6 ("pinctrl: stm32: Implement .get_direction
gpio_chip callback") exported the function stm32_pmx_get_mode()
and the struct stm32_gpio_bank, but these were never used outside
the file that defines them.

Declare the function as static, drop it from the include file and
drop also the struct, not anymore visible outside the file.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Link: https://lore.kernel.org/20250610143042.295376-2-antonio.borneo@foss.st.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>