[ROCm] Fix AITER MoE SITU v2 A8W4 env var name for Kimi-K3 (#788)

Rename AITER_SITUV2_A8W4 to VLLM_ROCM_USE_AITER_MOE_SITUV2_A8W4 in the Kimi-K3 AMD hardware override, matching the env flag vLLM's ROCm AITER MoE path actually reads.

Signed-off-by: seungrokj <seungrok.jung@amd.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>