platforms/traceswoasync: Add shims for _get_baudrate()

* command.c sees "platform.h" and SWO_UART/TRACEUART macros
  but not usart_get_baudrate() of <libopencm3/stm32/usart.h>
* traceswoasync.c also sees "platform.h" and usart.h, so
  create visible small helpers which resolve to correct function

src/platforms/common/stm32/traceswoasync.c

@@ -91,6 +91,11 @@ void trace_buf_drain(usbd_device *dev, uint8_t ep)
 	atomic_flag_clear_explicit(&reentry_flag, memory_order_relaxed);
 }
 
+uint32_t traceswo_get_baudrate(void)
+{
+	return usart_get_baudrate(SWO_UART);
+}
+
 void traceswo_setspeed(uint32_t baudrate)
 {
 	dma_disable_channel(SWO_DMA_BUS, SWO_DMA_CHAN);

src/platforms/common/stm32/traceswoasync_f723.c

@@ -75,6 +75,11 @@ void trace_buf_drain(usbd_device *dev, uint8_t ep)
 	atomic_flag_clear_explicit(&reentry_flag, memory_order_relaxed);
 }
 
+uint32_t traceswo_get_baudrate(void)
+{
+	return usart_get_baudrate(SWO_UART);
+}
+
 void traceswo_setspeed(uint32_t baudrate)
 {
 	dma_disable_stream(SWO_DMA_BUS, SWO_DMA_STREAM);

src/platforms/common/tm4c/traceswo.c

@@ -87,6 +87,11 @@ void traceswo_baud(unsigned int baud)
 	uart_set_databits(TRACEUART, 8);
 }
 
+uint32_t traceswo_get_baudrate(void)
+{
+	return uart_get_baudrate(TRACEUART);
+}
+
 #define FIFO_SIZE 256U
 
 /* RX Fifo buffer */

src/platforms/common/traceswo.h

@@ -28,6 +28,7 @@
 #define SWO_DEFAULT_BAUD 2250000U
 void traceswo_init(uint32_t baudrate, uint32_t swo_chan_bitmask);
 void traceswo_deinit(void);
+uint32_t traceswo_get_baudrate(void);
 #else
 void traceswo_init(uint32_t swo_chan_bitmask);
 #endif