drivers: dma_wch: Add __used instead of disabling unused function warning

#84065 replaced diagnostic pragmas with TOOLCHAIN_* macros, but we don't
need to use that here since __used is a cleaner way to indicate that the
function is used and will also prevent it from being optimized away at
link time if LTO is enabled.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>