http2: simplify onSelectPadding

`OnCallbackPadding` on the native side already clamps
the return value into the right range, so there’s not need
to also do that on the JS side.

Also, use `>>> 0` instead of `| 0` to get an uint32, since
the communication with C++ land happens through an Uint32Array.

PR-URL: https://github.com/nodejs/node/pull/17717


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>