tg3: fix to append hardware time stamping flags

The commit f233a976 (tg3: shows
HW time stamping support only if ptp_capable is present) didn't
append hardware flags correctly. This patch fixes it.
Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/tg3.c

@@ -5995,7 +5995,7 @@ static int tg3_get_ts_info(struct net_device *dev, struct ethtool_ts_info *info)
 				SOF_TIMESTAMPING_SOFTWARE;
 
 	if (tg3_flag(tp, PTP_CAPABLE)) {
-		info->so_timestamping = SOF_TIMESTAMPING_TX_HARDWARE |
+		info->so_timestamping |= SOF_TIMESTAMPING_TX_HARDWARE |
 					SOF_TIMESTAMPING_RX_HARDWARE |
 					SOF_TIMESTAMPING_RAW_HARDWARE;
 	}