device: use bitfields to keep DeviceStructure small

this keeps it small enough to fit in a float, which makes it possible
to see the full value in a MAVLink tlog

Conflicts:
	mavlink/include/mavlink/v1.0

src/drivers/device/device.h

@@ -134,11 +134,14 @@ public:
 	};
 
 	/*
-	  broken out 
+	  broken out device elements. The bitfields are used to keep
+	  the overall value small enough to fit in a float accurately,
+	  which makes it possible to transport over the MAVLink
+	  parameter protocol without loss of information.
 	 */
 	struct DeviceStructure {
-		enum DeviceBusType bus_type;
-		uint8_t bus;       // which instance of the bus type
+		enum DeviceBusType bus_type:3;
+		uint8_t bus:5;     // which instance of the bus type
 		uint8_t address;   // address on the bus (eg. I2C address)
 		uint8_t devtype;   // device class specific device type
 	};