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

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