net: dsa: mv88e6xxx: write MAC outside of ATU Get Next code

There is no need to write the MAC address before every Get Next
operation, since ATU MAC registers are not cleared between calls.

Move the _mv88e6xxx_atu_mac_write call outside of _mv88e6xxx_atu_getnext
so future code could call ATU Get Next multiple times and save a few
register access.

Signed-off-by: David S. Miller <davem@davemloft.net>