drm/amd/powerplay: rename smum header guards

This patch renames the smum header guards to align with the file name.

Reported-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>