sfc: move modparam 'rss_cpus' out of common channel code

Instead of exposing this old module parameter on the new driver (thus
 having to keep it forever after for compatibility), let's confine it
 to the old one; if we find later that we need the feature, we ought
 to support it properly, with ethtool set-channels.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>