drm/amdgpu/navi: fix index for OD MCLK

You can only adjust the max mclk, not the min.

Bug: https://gitlab.freedesktop.org/drm/amd/issues/1020
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 5.5.x