- Do not set an intermediate mode, prepare correct mode settings in a temporary
variable
- Consistently enabled the GPIO periph in gpio_init_af()
- Previously, STM32 F1 did not require a separate call to gpio_init() prior
to a call of gpio_init_af(), but other STM32 families did
- Now, gpio_init_af() can be used without gpio_init() consistently
- STM32 F1: Do not touch ODR for non input pins
- For input pins, this enables / disabled pull up resistors. For outputs,
this register should remain untouched (according to API doc)
