sfc: disable RSS when unsupported

When certain firmware variants are selected (via the sfboot utility) the
SFC7000 and SFC8000 series NICs don't support RSS. The driver still
tries (and fails) to insert filters with the RSS flag, and the NIC fails
to pass traffic.

When the firmware reports RSS_LIMITED suppress allocating a default RSS
context. The absence of an RSS context is picked up in filter insertion
and RSS flags are discarded.

Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>