net: stmmac: Fix TX timestamp calculation

When using GMAC4 the value written in PTP_SSIR should be shifted however
the shifted value is also used in subsequent calculations which results
in a bad timestamp value.
Signed-off-by: Fredrik Hallenberg <megahallon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -34,6 +34,7 @@ static u32 stmmac_config_sub_second_increment(void __iomem *ioaddr,
 {
 	u32 value = readl(ioaddr + PTP_TCR);
 	unsigned long data;
+	u32 reg_value;
 
 	/* For GMAC3.x, 4.x versions, convert the ptp_clock to nano second
 	 *	formula = (1/ptp_clock) * 1000000000
@@ -50,10 +51,11 @@ static u32 stmmac_config_sub_second_increment(void __iomem *ioaddr,
 
 	data &= PTP_SSIR_SSINC_MASK;
 
+	reg_value = data;
 	if (gmac4)
-		data = data << GMAC4_PTP_SSIR_SSINC_SHIFT;
+		reg_value <<= GMAC4_PTP_SSIR_SSINC_SHIFT;
 
-	writel(data, ioaddr + PTP_SSIR);
+	writel(reg_value, ioaddr + PTP_SSIR);
 
 	return data;
 }