sfxge: avoid TSO packets collapses if packet header uses 2 segments

It is really observed in the case of VLAN over sfxge interface.
Also this change makes total value equal to 35 which is default assumed
by the kernel for if_hw_tsomaxsegcount.

Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4319