net: dsa: mv88e6xxx: Restore USXGMII support for 6393X

In 4a562127

, USXGMII support was added for 6393X, but this was
lost in the PCS conversion (the blamed commit), most likely because
these efforts where more or less done in parallel.

Restore this feature by porting Michal's patch to fit the new
implementation.

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Tested-by: Michal Smulski <michal.smulski@ooma.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Fixes: e5b732a2

 ("net: dsa: mv88e6xxx: convert 88e639x to phylink_pcs")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Link: https://lore.kernel.org/r/20231205221359.3926018-1-tobias@waldekranz.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>