realtek: pcs: rtl93xx: configure in-band AN advertisement

realtek: pcs: rtl93xx: configure in-band AN advertisement

Enabling and disabling in-band auto-negotation is already supported on
RTL93xx. However, so far the advertisement is left unchanged at the
default of 0x1a0 (full duplex + pause + asymmetric pause).

Instead, set the advertisement to reflect the current configuration for
1000Base-X and 2500Base-X. Nothing needs to be done for SGMII, as the
advertisement register is ignored in that case.

Testing shows that negotiation of flow control works for 1000Base-X and
2500Base-X (tested with RTL930x on both ends of the link).

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Link: https://github.com/openwrt/openwrt/pull/21869
Signed-off-by: Robert Marko <robimarko@gmail.com>