Add accessor functions for the if_hw_tsomax* members of ifnet structure.

[stevek]

Summary

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.

Test Plan

This change has been used at Juniper for some time now.

[erj]

This matches the existing method for accessing fields in the rest of the if_t api, so I'm okay with it.