net: dsa: mv88e6xxx: extract ATU data write access

Other ATU commands need to write the ATU data register. To ease the
introduction of such commands, extract the ATU data write access from
_mv88e6xxx_atu_load to its own function.

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