[PATCH] sis190.c: fix multicast MAC filter

Here is a patch that changes the way the MAC filter is computed for the
multicast addresses. The computation is taken from the SiS GPL driver.

This patch is necessary to get IPv6 working.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>