hm: Fix ignored qualifiers warning

When building with -Wextra this warning popped up.  It's useless to
return a const bool here, because it's a copy anyway.  Instead for C++
const correctness a getter has the keyword in the back which means you
won't change your object when calling this method.

Fixes: 51fbe7868c ("added command queue")

src/hm/hmInverter.h

@@ -88,7 +88,7 @@ class CommandAbstract {
         };
         virtual ~CommandAbstract() {};
 
-        const uint8_t getCmd() {
+        uint8_t getCmd() const {
             return _Cmd;
         }