r8169: improve firmware handling

So far member rtl_fw has three states:
- IS_ERR(rtl_fw): firmware not loaded
- !rtl_fw: no firmware available
- other: firmware loaded

This can be made simpler and clearer by adding the firmware name as
member fw_name to struct rtl8169_private. Then:

- !fw_name: no firmware available
- !rtl_fw: firmware not loaded
- rtl_fw: firmware loaded

This change also allows to easily merge rtl_request_uncached_firmware
into rtl_request_firmware.

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