Fix SqueezeBERT for masked language model
# What does this PR do?
This corrects a mistake in the implementation of SqueezeBertForMaskedLM.

Fixes #8277


## Before submitting
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Yes: https://github.com/huggingface/transformers/issues/8277
- [x] Did you make sure to update the documentation with your changes? Here are the
- [ ] Did you write any new necessary tests? _No tests added._


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

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