sfc: Fix byte order warning in self-test

Add necessary cast when setting a bogus checksum.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/selftest.c b/drivers/net/ethernet/sfc/selftest.c
index ce72ae4..f6651d3 100644
--- a/drivers/net/ethernet/sfc/selftest.c
+++ b/drivers/net/ethernet/sfc/selftest.c
@@ -373,7 +373,7 @@
 	/* saddr set later and used as incrementing count */
 	payload->ip.daddr = htonl(INADDR_LOOPBACK);
 	payload->ip.ihl = 5;
-	payload->ip.check = htons(0xdead);
+	payload->ip.check = (__force __sum16) htons(0xdead);
 	payload->ip.tot_len = htons(sizeof(*payload) - sizeof(struct ethhdr));
 	payload->ip.version = IPVERSION;
 	payload->ip.protocol = IPPROTO_UDP;