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

[christiaanb]

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

clash-protocols/clash-protocols/src/Protocols/Df.hs

Line 908 in 23869c2

otpAck = Maybe.isJust maybePush

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

otpAck = amtLeft0 > 0