r8169: add helper rtl_csi_mod for accessing extended config space

Add a helper for the Realtek-specific mechanism for accessing extended
config space if native access isn't possible.
This avoids code duplication.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/b368fd91-57d7-4cb5-9342-98b4d8fe9aea@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>