[Fix] Fixes attribute error in MLA implementation (#3618)

### What this PR does / why we need it?
Corrects the attribute access for retrieving the device from `q_a_proj`
to `q_proj`. This prevents an `AttributeError` as `q_a_proj` does not
exist on the class instance.

### Does this PR introduce _any_ user-facing change?
None.

### How was this patch tested?
Need MLAPO tests.

- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0

---------

Signed-off-by: Yizhou Liu <liu_yizhou@outlook.com>

tests/e2e/singlecard/spec_decode_v1/test_v1_mtp_correctness.py

@@ -1,8 +1,5 @@
 from __future__ import annotations
 
-import os
-from unittest.mock import patch
-
 import pytest
 from vllm import SamplingParams
 from vllm.config import CompilationConfig, CUDAGraphMode
@@ -111,19 +108,3 @@ def test_mtp2_correctness_full_graph(
     model_name: str,
 ):
     mtp_correctness(sampling_config, model_name, 2, CUDAGraphMode.FULL)
-
-
-@patch.dict(os.environ, {"VLLM_ASCEND_ENABLE_MLAPO": "1"})
-def test_mtp_correctness_piecewise_graph_with_mlapo_kernel(
-    sampling_config: SamplingParams,
-    model_name: str,
-):
-    mtp_correctness(sampling_config, model_name, 1)
-
-
-@patch.dict(os.environ, {"VLLM_ASCEND_ENABLE_MLAPO": "1"})
-def test_mtp_correctness_full_graph_with_mlapo_kernel(
-    sampling_config: SamplingParams,
-    model_name: str,
-):
-    mtp_correctness(sampling_config, model_name, 1, CUDAGraphMode.FULL)