net: dsa: mv88e6xxx: enable STU methods for 6320 family

Commit c050f5e9 ("net: dsa: mv88e6xxx: Fill in STU support for all
supported chips") introduced STU methods, but did not add them to the
6320 family. Fix it.

Fixes: c050f5e9 ("net: dsa: mv88e6xxx: Fill in STU support for all supported chips")
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250317173250.28780-6-kabel@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>