drm/amdgpu: remove unneeded variable

Fix the following coccicheck review:
./drivers/gpu/drm/amd/pm/swsmu/smu11/sienna_cichlid_ppt.c:1174:14-18
:Unneeded variable

Remove unneeded variable used to store return value.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ran jianping <ran.jianping@zte.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>