Revert "e1000/rtl8139: update HMP NIC when every bit is written"

This reverts commit cd5be5829c1ce87aa6b3a7806524fac07ac9a757.
Digging into hardware specs shows this does not
actually make QEMU behave more like hardware:

There are valid arguments backed by the spec to indicate why the version
of e1000 prior to cd5be582 was more correct: the high byte actually
includes a valid bit, this is why all guests write it last.

For rtl8139 there's actually a separate undocumented valid bit, but we
don't implement it yet.

To summarize all the drivers we know about behave in one way
that allows us to make an assumption about write order and avoid
spurious, incorrect mac address updates to the monitor.

Let's stick to the tried heuristic for 1.7 and
possibly revisit for 1.8.

Reported-by: Vlad Yasevich <vyasevic@redhat.com>
Reviewed-by: Vlad Yasevich <vyasevic@redhat.com>
Cc: Amos Kong <akong@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
