- enable transmit checksum offload - sync em_receive_checksum() closer to the FreeBSD driver
- enable transmit checksum offload
- sync em_receive_checksum() closer to the FreeBSD driver

tested on 540, 541PI, 543, 544 and 546EB by djm@, marco@, brad@
and a few others.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars