sfxge: do not change CSUM_TSO when IFCAP_TSOx is changed

It is simply not required since the kernel checks corresponding
IFCAP_TSOx capability and CSUM_TSO in hw-assisted offloads.
Note that CSUM_TSO is two bits (CSUM_IP_TSO|CSUM_IP6_TSO) and both bits
are set in IPv4 and IPv6 mbufs.

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D2546