arch: arm: stm32f1: Shift GPIO pin configuration values only once

In the stm32_gpio_flags_to_conf function the configuration
values of the GPIO pin are shifted two times. One in the
stm32-pinctrlf1 header and one in the function. This patch
removes one of those shifts.

Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>