drivers/gpio: stm32: Change order of registers configuration

During implementation of i2c pinctrl configuration within i2c driver,
it appears that current order of register configuration used to
generate a spike on I2C bus, leading to broken configuration with
I2C device.
Reverse the order so that pin mode setting is done only after pupd,
speed and type are set, in order to avoid generating unwanted
artefacts on the bus.

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