net: stmmac: fix incorrect GMAC_VLAN_TAG register writting in GMAC4+

It should always do a read of current value of GMAC_VLAN_TAG instead of
directly overwriting the register value.

Fixes: c1be0022 ("net: stmmac: Add VLAN HASH filtering support in GMAC4+")
Signed-off-by: Tan, Tee Min <tee.min.tan@intel.com>
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwmac4_core.c

@@ -736,11 +736,14 @@ static void dwmac4_update_vlan_hash(struct mac_device_info *hw, u32 hash,
 				    __le16 perfect_match, bool is_double)
 {
 	void __iomem *ioaddr = hw->pcsr;
+	u32 value;
 
 	writel(hash, ioaddr + GMAC_VLAN_HASH_TABLE);
 
+	value = readl(ioaddr + GMAC_VLAN_TAG);
+
 	if (hash) {
-		u32 value = GMAC_VLAN_VTHM | GMAC_VLAN_ETV;
+		value |= GMAC_VLAN_VTHM | GMAC_VLAN_ETV;
 		if (is_double) {
 			value |= GMAC_VLAN_EDVLP;
 			value |= GMAC_VLAN_ESVL;
@@ -759,8 +762,6 @@ static void dwmac4_update_vlan_hash(struct mac_device_info *hw, u32 hash,
 
 		writel(value | perfect_match, ioaddr + GMAC_VLAN_TAG);
 	} else {
-		u32 value = readl(ioaddr + GMAC_VLAN_TAG);
-
 		value &= ~(GMAC_VLAN_VTHM | GMAC_VLAN_ETV);
 		value &= ~(GMAC_VLAN_EDVLP | GMAC_VLAN_ESVL);
 		value &= ~GMAC_VLAN_DOVLTC;