net: stmmac: xgmac: fix incorrect XGMAC_VLAN_TAG register writting

We should always do a read of current value of XGMAC_VLAN_TAG instead of
directly overwriting the register value.

Fixes: 3cd1cfcb ("net: stmmac: Implement VLAN Hash Filtering in XGMAC")
Signed-off-by: NOng Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwxgmac2_core.c

@@ -569,7 +569,9 @@ static void dwxgmac2_update_vlan_hash(struct mac_device_info *hw, u32 hash,
 
 		writel(value, ioaddr + XGMAC_PACKET_FILTER);
 
-		value = XGMAC_VLAN_VTHM | XGMAC_VLAN_ETV;
+		value = readl(ioaddr + XGMAC_VLAN_TAG);
+
+		value |= XGMAC_VLAN_VTHM | XGMAC_VLAN_ETV;
 		if (is_double) {
 			value |= XGMAC_VLAN_EDVLP;
 			value |= XGMAC_VLAN_ESVL;
@@ -584,7 +586,9 @@ static void dwxgmac2_update_vlan_hash(struct mac_device_info *hw, u32 hash,
 
 		writel(value, ioaddr + XGMAC_PACKET_FILTER);
 
-		value = XGMAC_VLAN_ETV;
+		value = readl(ioaddr + XGMAC_VLAN_TAG);
+
+		value |= XGMAC_VLAN_ETV;
 		if (is_double) {
 			value |= XGMAC_VLAN_EDVLP;
 			value |= XGMAC_VLAN_ESVL;