r8169: make rtl_fw_format_ok and rtl_fw_data_ok more independent

In preparation of factoring out the firmware handling code avoid any
usage of struct rtl8169_private internals. As part of it we can inline
rtl_check_firmware.

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