net: dsa: mv88e6xxx: rework ATU Flush

Add a fresh documented implementation of the ATU Flush/Move operation.
Use it to replace the current ATU Flush operation.

_mv88e6xxx_atu_flush_move is still used by the Move operation so keep it
until the Move operation is refactored in a next commit.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>