[Bugfix] rename enable_flash_comm_v1 back to enable_sp (#6883)

### What this PR does / why we need it?

PR #5632 introduced a bug by replacing some branches gated by enable_sp
with enable_flash_comm_v1. As a result, when enable_shared_expert_dp is
enabled alone (i.e., VLLM_ASCEND_ENABLE_FLASHCOMM1=0 and
VLLM_ASCEND_ENABLE_FLASHCOMM=0), the behavior becomes inconsistent with
the previous logic and leads to accuracy issues. This PR restores the
original enable_sp-based branching to recover expected behavior and
accuracy.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

#### 1. start server
``` bash
vllm serve /home/weights/DeepSeek-V2-Lite-W8A8/  \
    --port 8001 \
    --served-model-name auto \
    --max-model-len 1024 \
    --enforce-eager \
    --tensor-parallel-size 2 \
    --data-parallel-size 2 \
    --gpu-memory-utilization 0.9 \
    --enable-expert-parallel \
    --additional-config '{"enable_shared_expert_dp": true}'
```

#### 2. curl
```bash
curl -s http://localhost:8001/v1/chat/completions \
-H "Content-Type: application/json" \
-d '{
  "model": "auto",
  "messages": [
    {"role": "user", "content": "Hello. I have a question. Who are you?"}
  ],
  "max_tokens": 10,
  "temperature": 0.0,
  "ignore_eos_token": true
}'
```

- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

Signed-off-by: realliujiaxu <realliujiaxu@163.com>

vllm_ascend/platform.py

@@ -39,7 +39,6 @@ from vllm_ascend.utils import (
     COMPRESSED_TENSORS_METHOD,
     AscendDeviceType,
     check_kv_extra_config,
-    enable_sp,
     flashcomm2_enable,
     get_ascend_device_type,
     is_moe_model,
@@ -48,7 +47,7 @@ from vllm_ascend.utils import (
     update_aclgraph_sizes,
     update_cudagraph_capture_sizes,
     is_310p,
-    enable_flash_comm_v1,
+    enable_sp,
 )
 
 if TYPE_CHECKING:
@@ -402,7 +401,7 @@ class NPUPlatform(Platform):
             )
             vllm_config.parallel_config.cp_kv_cache_interleave_size = cache_config.block_size
 
-        if enable_flash_comm_v1():
+        if enable_sp(vllm_config):
             assert not is_vl_model(vllm_config), """Flash Comm V1 is not supported for VL models. \
                 Please disable it by setting VLLM_ASCEND_ENABLE_FLASHCOMM1=0. \
                 For optimal performance with VL models, we recommend enabling Sequence Parallelism \