DataFlash: correct consts on CardInserted

libraries/DataFlash/DataFlash_Backend.h

@@ -18,7 +18,7 @@ public:
 
     void internal_error();
 
-    virtual bool CardInserted(void) = 0;
+    virtual bool CardInserted(void) const = 0;
 
     // erase handling
     virtual void EraseAll() = 0;

libraries/DataFlash/DataFlash_File.cpp

@@ -214,7 +214,7 @@ uint32_t DataFlash_File::bufferspace_available()
 }
 
 // return true for CardInserted() if we successfully initialized
-bool DataFlash_File::CardInserted(void)
+bool DataFlash_File::CardInserted(void) const
 {
     return _initialised && !_open_error;
 }

libraries/DataFlash/DataFlash_File.h

@@ -33,7 +33,7 @@ public:
 
     // initialisation
     void Init() override;
-    bool CardInserted(void) override;
+    bool CardInserted(void) const override;
 
     // erase handling
     void EraseAll() override;

libraries/DataFlash/DataFlash_MAVLink.h

@@ -40,7 +40,7 @@ public:
                                bool is_critical) override;
 
     // initialisation
-    bool CardInserted(void) override { return true; }
+    bool CardInserted(void) const override { return true; }
 
     // erase handling
     void EraseAll() override {}