sfc: Restore the effect of the rss_cpus module parameter

Commit a4900ac9 ("sfc: Create multiple TX queues") accidentally
disabled the rss_cpus module parameter.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

drivers/net/sfc/efx.c

@@ -1153,6 +1153,9 @@ static int efx_wanted_channels(void)
 	int count;
 	int cpu;
 
+	if (rss_cpus)
+		return rss_cpus;
+
 	if (unlikely(!zalloc_cpumask_var(&core_mask, GFP_KERNEL))) {
 		printk(KERN_WARNING
 		       "sfc: RSS disabled due to allocation failure\n");