crypto: return `this` in setAuthTag/setAAD

Allow method chaining as with setAutoPadding and other methods.

PR-URL: https://github.com/nodejs/node/pull/9398


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sam Roberts <sam@strongloop.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>