[netdrvr] b44: handle excessive multicast groups

If there are more than B44_MCAST_TABLE_SIZE groups in the dev->mc_list,
it will only listen to the first B44_MCAST_TABLE_SIZE that it sees.

This change makes the driver go into RXCONFIG_ALLMULTI mode if there
are more than B44_MCAST_TABLE_SIZE groups being subscribed to, similar
to other network drivers.

Noticed by Bill Helfinstine <bhelf@flitterfly.whirpon.com>

Signed-off-by: Jeff Garzik <jeff@garzik.org>