r8169: add RTL_GIGA_MAC_VER_LAST to facilitate adding support for new chip versions

Add a new mac_version enum value RTL_GIGA_MAC_VER_LAST. Benefit is that
when adding support for a new chip version we have to touch less code,
except something changes fundamentally.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/06991f47-2aec-4aa2-8918-2c6e79332303@gmail.com


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