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>