phy_sanitize_settings() is supposed to pick the least supported
speed from the supported list when speed is not initialized.

What makes you think it should pick the slowest speed? The kdoc for
the function is:

/**
 * phy_sanitize_settings - make sure the PHY is set to supported speed and duplex
 * @phydev: the target phy_device struct
 *
 * Description: Make sure the PHY is set to supported speeds and
 *   duplexes.  Drop down by one in this order:  1000/FULL,
 *   1000/HALF, 100/FULL, 100/HALF, 10/FULL, 10/HALF.

So it should pick 1000Full if available. If not it will try 1000Half,
if not 100Full etc.

And the comment is actually a bit out of date. It will actually start
from 800G Full, 400G Full, 200G Full, 100G Full, not that anybody does
Copper at these speeds.

	Andrew