pinctrl: stm32: stm32: Add of_node_put() before return

Each iteration of for_each_child_of_node and
for_each_available_child_of_node puts the previous node, but in
the case of a return from the middle of the loop, there is no put, thus
causing a memory leak. Hence add an of_node_put before the return in
two places.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Link: https://lore.kernel.org/r/20190808075457.16109-1-nishkadg.linux@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>