e1000: Clean up legacy absolute and packet timers

The absolute and packet timers only apply to lem and em with some only
applying to the later.

This cleans up the sysctl tree to only show these where applicable and
stops writing to unexpected registers for igb.

Sponsored by: BBOX.io

(cherry picked from commit 1c578f1c93112d3d53820f8b6b411488b83f0ab2)