Build Realtek Gigabit Ethernet driver as a module.

Network cards based on chipsets this driver supports are ubiquitous both
in regular PCIe as well as mini-PCIe and nowadays even various M.2
formats. It is therefore a suitable card to be used for any kind of PCIe
and/or Gigabit Ethernet testing. As it is not designed in, just enabling
it as a module seems most suitable.
Signed-off-by: NMarcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: NKrzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>