sfc: Restore the effect of the rss_cpus module parameter

Commit a4900ac ("sfc: Create multiple TX queues") accidentally
disabled the rss_cpus module parameter.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/sfc/efx.c b/drivers/net/sfc/efx.c
index c2dc9a5..002bac7 100644
--- a/drivers/net/sfc/efx.c
+++ b/drivers/net/sfc/efx.c
@@ -1153,6 +1153,9 @@
 	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");