On Siena all received packets that don't match a more specific filter
will match the unicast or multicast default filter.  Currently we
leave these set to the default values (RSS with base queue number of
0).  Allow them to be reconfigured to select a single RX queue.

These default filters are programmed through the FILTER_CTL register,
but we represent them internally as an additional table of size 2.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>