pinctrl: st: Move st_get_pio_control() further up the source file

st_get_pio_control() will be used by subsequent calls which are
to be located above its original position.  This is required to
prevent the need for an unnecessary forward-declaration/prototype.

Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>