fix error async_scheduler can't be enabled (#3127)

### What this PR does / why we need it?
PR #2894 make ascend_scheduler_config.enabled always be `True` for
non-mla models，when `ascend_scheduler_config.enabled=True `, it will
always initialize `AscendScheduler` which is a subclass of `Scheduler`,
but when we enbale async_scheduling,we need to initialize
`AsyncScheduler` in vllm, this will make async_scheduling can't be
enabled.

### Does this PR introduce _any_ user-facing change?
not-related

### How was this patch tested?
when user set `async_scheduling`, it means user don't want to use
`AscendScheduler`, so we shouldn't set `ascend_scheduler_config.enabled
= True`

- vLLM version: v0.10.2
- vLLM main:
vllm-project/vllm@f225ea7

Signed-off-by: Ronald1995 <[email protected]>

Author: Ronald1995

vllm_ascend/platform.py

@@ -135,7 +135,8 @@ def check_and_update_config(cls, vllm_config: VllmConfig) -> None:
         else:
             structured_outputs_config = vllm_config.structured_outputs_config
 
-        if model_config is not None and not model_config.use_mla:
+        if (model_config is not None and not model_config.use_mla
+                and not scheduler_config.async_scheduling):
             logger.info(
                 "Non-MLA LLMs forcibly disable the chunked prefill feature,"
                 "as the performance of operators supporting this feature "