net: stmmac: dwmac1000: provide multicast filter fallback

If we don't have a hardware multicast filter available then instead of
silently failing to listen for the requested ethernet broadcast
addresses fall back to receiving all multicast packets, in a similar
fashion to other drivers with no multicast filter.

Cc: stable@vger.kernel.org
Signed-off-by: Jonathan McDowell <noodles@earth.li>
Signed-off-by: David S. Miller <davem@davemloft.net>