r8169: remove not needed check in rtl_fw_write_firmware

This check can never be true for a firmware file with a correct format.
Existing checks in rtl_fw_data_ok() are sufficient, no problems with
invalid firmware files are known.

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