decklink: Fix compiler warning about ignored const

Fix a warning from gcc about one of the const keywords being ignored.
This happens because the returned type is not a reference contrary to
the methods above.

plugins/decklink/decklink-device.cpp

@@ -128,7 +128,7 @@ const std::string& DeckLinkDevice::GetName(void) const
 	return name;
 }
 
-const int32_t DeckLinkDevice::GetMaxChannel(void) const
+int32_t DeckLinkDevice::GetMaxChannel(void) const
 {
 	return maxChannel;
 }

plugins/decklink/decklink-device.hpp

@@ -31,7 +31,7 @@ public:
 	const std::string& GetHash(void) const;
 	const std::vector<DeckLinkDeviceMode *>& GetModes(void) const;
 	const std::string& GetName(void) const;
-	const int32_t GetMaxChannel(void) const;
+	int32_t GetMaxChannel(void) const;
 
 	bool GetInput(IDeckLinkInput **input);