sfc: Fix memcpy() with const destination compiler warning.

drivers/net/ethernet/sfc/selftest.c: In function ‘efx_iterate_state’:
drivers/net/ethernet/sfc/selftest.c:388:9: warning: passing argument 1 of ‘memcpy’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-array-qualifiers]

This is because the msg[] member of struct efx_loopback_payload
is marked as 'const'.  Remove that.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/selftest.c

@@ -46,7 +46,7 @@ struct efx_loopback_payload {
 	struct iphdr ip;
 	struct udphdr udp;
 	__be16 iteration;
-	const char msg[64];
+	char msg[64];
 } __packed;
 
 /* Loopback test source MAC address */