r8169: use macro FIELD_SIZEOF in definition of FW_OPCODE_SIZE

Using macro FIELD_SIZEOF makes this define easier understandable.
Signed-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/r8169_firmware.c

@@ -37,7 +37,7 @@ struct fw_info {
 	u8	chksum;
 } __packed;
 
-#define FW_OPCODE_SIZE	sizeof(typeof(*((struct rtl_fw_phy_action *)0)->code))
+#define FW_OPCODE_SIZE FIELD_SIZEOF(struct rtl_fw_phy_action, code[0])
 
 static bool rtl_fw_format_ok(struct rtl_fw *rtl_fw)
 {