Changes to ip_eth_tx_64.v

[EdwinEstep]

After the last commit to ip_eth_tx_64.v, it no longer passes its MyHDL unit test. Specifically, it fails test 5 (but only with backpressure applied): #203. This only occurs after the latest commit on that file, which was to "Rewrite early ready condition". From what I can tell, the change was made to optimize the code, simplifying the combinatorial paths, rather than to fix a bug. I noticed that this change was copy-pasted across multiple modules, which could explain why this one was overlooked.

@alexforencich Would it make sense to just revert the change? Was it needed to reduce the design to fit the constraints of some example designs?