llama : fix incorrect model type for Gemma 270M (#15764)

llama : fix incorrect model type for  Gemma 270M (#15764)

This commit fixes the model type for the Gemma 270M model in
llama_model.cpp which should be LLM_TYPE_270M. I incorrectly added this
previously as LLM_TYPE_537M which was wrong.

The motivation for this is that it causes the model to not be identified
properly when using tools like llama-bench. For example:
```console
$ ./build/bin/llama-bench -m models/gemma-3-270m-Q8_0.gguf
| model                          |       size | ...
| ------------------------------ | ---------: | ...
| gemma3 ?B Q8_0                 | 271.81 MiB | ...
| gemma3 ?B Q8_0                 | 271.81 MiB | ...
```

With the changes in this commit the output will be:
```console
$ ./build/bin/llama-bench -m models/gemma-3-270m-Q8_0.gguf
| model                          |       size | ...
| ------------------------------ | ---------: | ...
| gemma3 270M Q8_0               | 271.81 MiB | ...
| gemma3 270M Q8_0               | 271.81 MiB | ...
```