drm/amdgpu: use %u rather than %d for sclk/mclk

Large clock values may overflow and show up as negative.

Reported by prOMiNd on IRC.

Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org