igb: Code to prevent overwriting SFP I2C

This patch fixes "overwrite" problem. without this fix, SFP I2C EEPROM
data, which is located at A0 can be overwritten by the phy write function.
Signed-off-by: "Akeem G. Abodunrin" <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/e1000_phy.c

@@ -306,6 +306,12 @@ s32 igb_write_phy_reg_i2c(struct e1000_hw *hw, u32 offset, u16 data)
 	u32 i, i2ccmd = 0;
 	u16 phy_data_swapped;
 
+	/* Prevent overwritting SFP I2C EEPROM which is at A0 address.*/
+	if ((hw->phy.addr == 0) || (hw->phy.addr > 7)) {
+		hw_dbg("PHY I2C Address %d is out of range.\n",
+			  hw->phy.addr);
+		return -E1000_ERR_CONFIG;
+	}
 
 	/* Swap the data bytes for the I2C interface */
 	phy_data_swapped = ((data >> 8) & 0x00FF) | ((data << 8) & 0xFF00);