NET: am79c961: ensure multicast filter is correctly set at open

We were clearing out the multicast filter whenever the interface was
upped, and not setting the mode bits correctly.  This can cause
problems if there are any multicast addresses already set at this
point, or if ALLMULTI was set.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>