pinctrl: st: fix leaked of_node references

The call to of_get_child_by_name returns a node pointer with refcount
incremented thus it must be explicitly decremented after the last
usage.

Detected by coccinelle with the following warnings:
./drivers/pinctrl/pinctrl-st.c:1188:3-9: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 1175, but without a corresponding object release within this function.
./drivers/pinctrl/pinctrl-st.c:1188:3-9: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 1175, but without a corresponding object release within this function.
./drivers/pinctrl/pinctrl-st.c:1199:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 1175, but without a corresponding object release within this function.
./drivers/pinctrl/pinctrl-st.c:1199:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 1175, but without a corresponding object release within this function.

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Cc: linux-kernel@vger.kernel.org (open list)
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>