Fix small field arith machine for bus linker mode

[georgwiese]

Same change as #2297. I opened #2340 to track fixing the root cause.

With this change, we can have the BabyBear RISC-V machine with a bus:

cargo run -r --features plonky3 --bin powdr-rs compile riscv/tests/riscv_data/keccak-o output --field bb
cargo run -r --features plonky3 pil output/keccak.asm -o output -f --field bb --prove-with plonky3 --linker-mode bus

[leonardoalt]

witgen doesn't like it

[georgwiese]

Oh wow, it was actually a copy-paste error (the range check went to byte2 instead of byte). Fixed now.