Fix some minor TSO issues:
- Improve description of TSO limits.
- Remove a not needed KASSERT()
- Remove some not needed variable casts.

Sponsored by:	Mellanox Technologies
Discussed with:	lstewart @
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274376 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f