realtek: adapt RTl931x "digital 2" serdes page calculation

realtek: adapt RTl931x "digital 2" serdes page calculation

The more we step down into the SerDes deeps the more confusing it
gets. Nevertheless it is not to late to fix a wrong assumption.
Until now it seemed as if the frontend/backend SerDes mapping is
totally without intersection. This is not true.

The backend SerDes mapping is also dependent on the mode. Especially
the proprietary Realtek XSGMII mode stands out from all other
mappings. So fix the descriptions and the calculation of the third
page package (digital 2 aka XSGMII 2).

As it was not yet used it had no impact.

Fixes: a4cbb44c1b ("realtek: convert access to RTL931x analog serdes pages")
Signed-off-by: Markus Stockhausen <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19873
Signed-off-by: Hauke Mehrtens <[email protected]>