r8169: add rtl_fw_request_firmware and rtl_fw_release_firmware

Add rtl_fw_request_firmware and rtl_fw_release_firmware which will be
part of the API when factoring out the firmware handling code.

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