net: dsa: mv88e6xxx: rename ATU MAC accessors

Rename the __mv88e6xxx_{read,write}_addr functions to more explicit
_mv88e6xxx_atu_mac_{read,write} functions, which also respect the single
underscore convention used in the file (meaning SMI lock must be held).

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