`Df.fifo` has unnecessary longer combinational path on ready

Df.fifo has an ack/ready that depends on the the state of the fifo and the input to the fifo:

https://github.com/clash-lang/clash-protocols/blob/23869c2345cb9dd296602b0b42b6de85b25c25c8/clash-protocols/src/Protocols/Df.hs#L908

We can make it just depend on the state while remaining protocol compliant, making for a shorter combinational path for complete systems, by doing:

``` haskell
otpAck = amtLeft0 > 0
```

Start at the linked Df.fifo implementation in clash-protocols/src/Protocols/Df.hs around line 908. Inspect how ack/ready depends on FIFO state and input, then verify that the proposed state-only dependency remains protocol compliant. Done means the combinational path is shortened without changing FIFO behavior.

Written by the indexing model from the issue text.