Backport of #14426

Contribution description

cpu/stm32: Fix gpio_init() / gpio_int_af()

• 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)

cpu/stm32: Fix uart_init()

• Make use of the fact that gpio_init_af() does not need prior call to gpio_init() for all STM32 families anymore and drop call to gpio_init()
• Initialize the UART periph first, before initializing the pins
  • While uninitialized, the UART periph will send signal LOW to TXD. This results in a start bit being picked up by the other side.
  • Instead, we do not connect the UART periph to the pins until it is initialized, so that the TXD level will already be HIGH when the pins are attached.
  • This results in no more garbage being send during initialization

Testing procedure

• UART should still work for STM32F1 and STM32Fx (x != 1)
• gpio_init() should still work for ST32F1 and STM32Fx (x != 1)

Note: It should be sufficient to test with STM32F1 and one of the other STM32 not belonging to the STM32F1 family

Issues/PRs references

Fixes #8045