It turns out that 2500Base-X actually works fine with in-band status on
MediaTek's LynxI PCS -- I wrongly concluded it didn't because it is
broken in all the copper SFP modules and GPON sticks I used for testing.
Hence report LINK_INBAND_ENABLE also for 2500Base-X mode.
This reverts most of commit a003c38d9bbb ("net: pcs: pcs-mtk-lynxi:
correctly report in-band status capabilities").
The removal of the QSGMII interface mode was correct and is left
untouched.