e1000: Automask TSO on lem(4)/em(4) 10/100 Ethernet

This feature masks TSO capability when a link comes up at 10 or 100mbit
due to errata on the chips. This behavior matches previous versions of
FreeBSD as well as NetBSD and Linux.

A tunable, hw.em.unsupported_tso may be set if the admin desires to
disabling automasking and configure TSO settings manually.

Differential Revision: https://reviews.freebsd.org/D41170

(cherry picked from commit 2ddf24f8f52559b08da700ab044c33d0a9ed5601)