r8169: provide some firmware information via ethtool.

There is no real firmware version yet but the manpage of ethtool
is rather terse about the driver information.

Former output:
$ ethtool -i eth1
driver: r8169
version: 2.3LK-NAPI
firmware-version:
bus-info: 0000:01:00.0
$ ethtool -i eth0
driver: r8169
version: 2.3LK-NAPI
firmware-version:
bus-info: 0000:03:00.0

Current output:
$ ethtool -i eth1
driver: r8169
version: 2.3LK-NAPI
firmware-version: N/A
bus-info: 0000:01:00.0

$ ethtool -i eth0
driver: r8169
version: 2.3LK-NAPI
firmware-version: rtl_nic/rtl8168d-1.fw
bus-info: 0000:03:00.0

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Fixed-by Ciprian Docan <docan@eden.rutgers.edu>
Cc: Realtek linux nic maintainers <nic_swsd@realtek.com>
Cc: Fejes József <fejes@joco.name>
Cc: Borislav Petkov <borislav.petkov@amd.com>