fix compiler warnings

firmware/hackrf_usb/hackrf_usb.c

@@ -266,6 +266,8 @@ int main(void) {
 		case TRANSCEIVER_MODE_CPLD_UPDATE:
 			cpld_update();
 			break;
+		default:
+			break;
 		}
 	}
 

firmware/hackrf_usb/usb_api_transceiver.c

@@ -36,6 +36,7 @@
 #include <usb_queue.h>
 
 #include <stddef.h>
+#include <string.h>
 
 #include "usb_endpoint.h"
 #include "usb_api_sweep.h"

firmware/hackrf_usb/usb_api_transceiver.h

@@ -59,5 +59,7 @@ usb_request_status_t usb_vendor_request_set_hw_sync_mode(
 transceiver_mode_t transceiver_mode(void);
 void set_transceiver_mode(const transceiver_mode_t new_transceiver_mode);
 void start_streaming_on_hw_sync();
+void rx_mode(void);
+void tx_mode(void);
 
 #endif/*__USB_API_TRANSCEIVER_H__*/