net: dsa: mv88e6xxx: remove debugfs interface

It is preferable to have a common debugfs interface for DSA or switchdev
instead of a driver specific one. Thus remove the mv88e6xxx debug code.

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