sfxge: relax assertion to allow RST flag in TSO packets

Kernel under stress load, mixed MC reboot and sfupdate really
generates TSO packet with RST flag.
It will generate many TCP packets with RST flag set.
May be RST flag should be set in the last segment only, but it could be
dropped. So, it is safer to keep the flag in all packets to be sure that
connection is reset.

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D2609