[U(S)ART] Fix AF settings

Since e02bc35, some serial
was not functional.
Ex for Nucleo-L432KC, the AF of USART2 (used for STLink VCP)
are not the same.

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/stm32/uart.c

@@ -285,6 +285,9 @@ void uart_init(serial_t *obj)
   GPIO_InitStruct.Pin         = STM_GPIO_PIN(obj->pin_tx);
   GPIO_InitStruct.Mode        = STM_PIN_MODE(function);
   GPIO_InitStruct.Pull        = STM_PIN_PUPD(function);
+#ifndef STM32F1xx
+  GPIO_InitStruct.Alternate   = STM_PIN_AFNUM(function);
+#endif /* STM32F1xx */
   HAL_GPIO_Init(port, &GPIO_InitStruct);