Add IFCAP_TSO6 for igb

It seems igb supports TSO6, but the capability got lost in
the iflib update. Restore this capability.

PR: 231476
Reported by: lev
Reviewed by: erj
Approved by: re (gjb)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D17242