make RSV flag a little bit more efficient

Hi

I replaced your check, if the RSV bits are set correctly by one, which is a little bit more efficient. You can check, that it is even the same, by a snippet of code:
```javascript
const RSV1        = 0x40;
const RSV2        = 0x20;
const RSV3        = 0x10;
const RSV4        = RSV1 | RSV2 | RSV3;
for (i=0;i<1000000;i++) {
	if ((RSV4 & i) != ((RSV1 & i | RSV2 & i | RSV3 & i))) {
		console.log((RSV4 & i) , (RSV1 & i | RSV2 & i | RSV3 & i), i);
	}
}
```

All checks passed.
I had to create a new Pull Request. Forgot, to create the branch. Hope now it is able to merge.

best regards Thomas