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

cpu/stm32/periph/uart.c

@@ -117,9 +117,6 @@ static inline void uart_init_cts_pin(uart_t uart)
 static inline void uart_init_pins(uart_t uart, uart_rx_cb_t rx_cb)
 {
      /* configure TX pin */
-    gpio_init(uart_config[uart].tx_pin, GPIO_OUT);
-    /* set TX pin high to avoid garbage during further initialization */
-    gpio_set(uart_config[uart].tx_pin);
 #ifdef CPU_FAM_STM32F1
     gpio_init_af(uart_config[uart].tx_pin, GPIO_AF_OUT_PP);
 #else
@@ -172,8 +169,6 @@ int uart_init(uart_t uart, uint32_t baudrate, uart_rx_cb_t rx_cb, void *arg)
     tsrb_init(&uart_tx_rb[uart], uart_tx_rb_buf[uart], UART_TXBUF_SIZE);
 #endif
 
-    uart_init_pins(uart, rx_cb);
-
     uart_enable_clock(uart);
 
     /* reset UART configuration -> defaults to 8N1 mode */
@@ -199,6 +194,12 @@ int uart_init(uart_t uart, uint32_t baudrate, uart_rx_cb_t rx_cb, void *arg)
     uart_init_usart(uart, baudrate);
 #endif
 
+    /* Attach pins to enabled UART periph. Note: It is important that the UART
+     * interface is configured prior to attaching the pins, as otherwise the
+     * signal level flickers during initialization resulting in garbage being
+     * sent. */
+    uart_init_pins(uart, rx_cb);
+
     /* enable RX interrupt if applicable */
     if (rx_cb) {
         NVIC_EnableIRQ(uart_config[uart].irqn);