sfc: limit the number of receive queues

The number of rx queues is determined by the rss_cpus parameter
or the cpu topology. If that is higher than EFX_MAX_RX_QUEUES the
driver can corrupt state.

Fixes: 8ceee660

 ("New driver "sfc" for Solarstorm SFC4000 controller.")
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>