Normalize preprocessor endif comments

Clean up `#endif` trailing comments across GD32 DMX sources to use consistent macro names without `defined(...)`/`!defined(...)` wrappers, and add missing clarifying comments in `dmx_internal.h`. Also fixes a typo in a TODO (`constepr` -> `constexpr`) and renames template parameters in GPIO helpers to match the project’s `k...` naming style.

Author: vanvught