AP_HAL: add FilteredCAN mode to tell driver to init as such

libraries/AP_HAL/CANIface.h

@@ -94,9 +94,12 @@ public:
     enum OperatingMode {
         PassThroughMode,
         NormalMode,
-        SilentMode
+        SilentMode,
+        FilteredMode
     };
 
+    OperatingMode get_operating_mode() { return mode_; }
+
     typedef uint16_t CanIOFlags;
     static const CanIOFlags Loopback = 1;
     static const CanIOFlags AbortOnError = 2;
@@ -206,4 +209,7 @@ public:
 
     // return true if init was called and successful
     virtual bool is_initialized() const = 0;
+protected:
+    uint32_t bitrate_;
+    OperatingMode mode_;
 };