realtek: pcs: rtl93xx: handle XSGMII autoneg

realtek: pcs: rtl93xx: handle XSGMII autoneg

The XSGMII mode is special in several regards. The inband
autonegotiation for this mode is called 'XSG N-way'. It is controlled
using different bits and location, and using XSG operations.

Add support for this by enhancing the set_autoneg implementation shared
by RTL930x and RTL931x. This can stay shared since it works the same for
both variants.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21762
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>