sfc: Cleanup RX queue information

Rename efx_nic::rss_queues to the more obvious n_rx_queues

Remove efx_rx_queue::used and other stuff that's redundant with it.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>