drm/amd/powerplay: drop unnecessary "@" on OD sysfs output

To follow conventional style. And this unnecessary "@" confuses
our userspace tool.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>