vtnet: improve control of transmit offloading

Keep the hwassist flags for transmit checksum offload and transmit
segment offload in sync with the enabled capabilities.

Reported by: Timo Völker
Reviewed by: Timo Völker
Differential Revision: https://reviews.freebsd.org/D52765

(cherry picked from commit f2575d56c8c9a8acad4a61a3586546dff4febce1)