USB on both MT7621 and MT7688 seems to work much better without doing slew rate calibration.
These are the only two SoCs, apart from MT7628, which actually make use of the slew rate calibration routines implemented in the mtk_usb_phy driver. Since MT7628 is actually a superset of MT7688 things should be the same for it as well.
We do not remove the code, we simply define it out.