drm/amd/pp: fix typecast error in powerplay.

commit 8d8258bdab735d9f3c4b78e091ecfbb2b2b1f2ca upstream.

resulted in unexpected data truncation

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>