r8169: add enum rtl_fw_opcode

Replace the firmware opcode defines with a proper enum. The BUG()
in rtl_fw_write_firmware() can be removed because the call to
rtl_fw_data_ok() ensures all opcodes are valid.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>