net: Add missing netdev feature strings for NETIF_F_HW_VLAN_STAG_*

Noticed by Ben Hutchings.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/ethtool.c

@@ -64,6 +64,9 @@ static const char netdev_features_strings[NETDEV_FEATURE_COUNT][ETH_GSTRING_LEN]
 
 	[NETIF_F_HW_VLAN_CTAG_RX_BIT] =  "rx-vlan-ctag-hw-parse",
 	[NETIF_F_HW_VLAN_CTAG_FILTER_BIT] = "rx-vlan-ctag-filter",
+	[NETIF_F_HW_VLAN_STAG_TX_BIT] =  "tx-vlan-stag-hw-insert",
+	[NETIF_F_HW_VLAN_STAG_RX_BIT] =  "rx-vlan-stag-hw-parse",
+	[NETIF_F_HW_VLAN_STAG_FILTER_BIT] = "rx-vlan-stag-filter",
 	[NETIF_F_VLAN_CHALLENGED_BIT] =  "vlan-challenged",
 	[NETIF_F_GSO_BIT] =              "tx-generic-segmentation",
 	[NETIF_F_LLTX_BIT] =             "tx-lockless",