smsc911x: enforce read-after-write timing restriction on eeprom access

The LAN911x datasheet specifies a minimum delay of 45ns between a write
of E2P_DATA and any read.  This patch adds a single dummy read of
BYTE_TEST to enforce this timing constraint.
Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/smsc911x.c

@@ -1680,6 +1680,7 @@ static int smsc911x_eeprom_write_location(struct smsc911x_data *pdata,
 					  u8 address, u8 data)
 {
 	u32 op = E2P_CMD_EPC_CMD_ERASE_ | address;
+	u32 temp;
 	int ret;
 
 	SMSC_TRACE(DRV, "address 0x%x, data 0x%x", address, data);
@@ -1688,6 +1689,10 @@ static int smsc911x_eeprom_write_location(struct smsc911x_data *pdata,
 	if (!ret) {
 		op = E2P_CMD_EPC_CMD_WRITE_ | address;
 		smsc911x_reg_write(pdata, E2P_DATA, (u32)data);
+
+		/* Workaround for hardware read-after-write restriction */
+		temp = smsc911x_reg_read(pdata, BYTE_TEST);
+
 		ret = smsc911x_eeprom_send_cmd(pdata, op);
 	}