Revert "lwip: Make the checksum faster."
Revert "lwip: Make the checksum faster."

This reverts commit 48dcb3e73c372f64881b6fa46f671f9cc4ed86b1.

ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,ambrop72/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,johnkingsley/aprinter