fix(runtime): reset token length after token creation

The patch f362213e moved attestation state management inside the
component. But the realm token length was not reset on successful
token creation which allowed another ATTEST_TOKEN_CONTINUE call to
go through. This patch fixes the issue.

Change-Id: If24994b29d2288fcdd85755cc53a23fcf7fa3882
Signed-off-by: Soby Mathew <soby.mathew@arm.com>