drm/amdgpu: fix debugfs creation/removal, again

There is still a warning when CONFIG_DEBUG_FS is disabled:

drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c:1145:13: error: 'amdgpu_ras_debugfs_create_ctrl_node' defined but not used [-Werror=unused-function]
 1145 | static void amdgpu_ras_debugfs_create_ctrl_node(struct amdgpu_device *adev)

Change the code again to make the compiler actually drop
this code but not warn about it.

Fixes: ae2bf61f ("drm/amdgpu: guard ras debugfs creation/removal based on CONFIG_DEBUG_FS")
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>