drivers/gpio: stm32: Rework configure function exit for dual core

With dual core handling introduction, we now need to take care to
always release lock before exiting function.
Rework gpio_stm32_config to take this into account.
Additionally, since ENOSYS usage is resevred to system calls
handling, replace with EIO.


Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>