sfxge: switch to TxQ creation specific flags
It is better do not mix TxQ creation and receive event flags since only
checksum flags are applicable to TxQ.
Also it will allow to add a new TxQ creation specific flags.
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4389