ure: improve transmit checksum offloading

Apparently, the name of the variable l4off was correct. Providing the
offset to the TCP or UDP header allows the transmit checksum offload to
work for TCP/IPv4, TCP/IPv6, UDP/IPv4, and UDP/IPv6.

Reported by: vishwin
Reviewed by: vishwin
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55187