Add the folowing set accessor functions for recently-added members of ifnet
structure:

if_gethwtsomax(), if_sethwtsomax() - if_hw_tsomax
if_gethwtsomaxsegcount(), if_sethwtsomaxsegcount() - if_hw_tsomaxsegcount
if_gethwtsomaxsegsize(), if_sethwtsomaxsegsize() - if_hw_tsomaxsegsize

Update em and vnic drivers which had already been coverted to use accessor
functions for the other ifnet structure members.

Reviewed by: erj
Approved by: sjg (mentor)
Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D8544