Avoid trying to toggle TSO twice

Remove TSO from the toggle mask when automatically disabled by TXCKSUM* in
various NIC drivers.

Reviewed by: hselasky, np, gallatin, jpaetzel
Approved by: mav (mentor)
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D25120