eepro100: Fix multicast regression

eepro100: Fix multicast regression

Commit 7fc8d918b9674c3e9233d6d25da2457345d414a0 removed code from
eepro100.c and replaced it by different code: the code in net.c
returns bits 31...26, but eepro100 needs bits 7...2.

This patch partially reverts 7fc8d918b9674c3e9233d6d25da2457345d414a0.
To avoid future problems, I renamed the function and changed the comment.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>