tpm2: ensure auth session contexts are flushed after use

[jakogut]

The TPM is capable of storing a limited number of auth session handles. Ensure auth sessions are flushed after use, to prevent tpm2_startauthsession from failing with 'out of session handles'.

Change-type: patch

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Contributor checklist

• [ ] Changes have been tested
  • [ ] Covered in automated test suite
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[jakogut]

@resin-jenkins retest this please