Transmit segment offloading depends on transmit checksum offloading. Enforce that constraint.
This also fixes a bug, since `if_hwassist` bits are from the `CSUM_` space, not from the `IFCAP_` space.
This should fix [[ https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=290773 | PR 290773 ]].