net: dsa: mv88e6xxx: do not increment bus refcount

The MDIO device probe and remove functions are respectively incrementing
and decrementing the bus refcount themselves. Since these bus level
actions are out of the device scope, remove them.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>