dw1000: added forgotten return value definitions in dw1000.h

Inc/dw1000.h

@@ -153,3 +153,9 @@ void vTaskGlobalTimeIncrement(void *pvParameters);
 void dw1000_generateConfig(uint16_t mode, dwt_config_t *config);
 
 #endif /*__ dw1000_H */
+/**
+ *	Defines various return codes for driver functions
+ */
+#define DW1000_RET_OK 0
+#define DW1000_RET_MSG_TO_LONG -1
+#define DW1000_RET_TX_BUFFER_OVERFLOW -2