net: dsa: mv88e6xxx: rename _mv88e6xxx_wait

Now that there is no locked version of the wait routine anymore, rename
the _ prefixed version and make it use the new read API.

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