dsa: mv88e6xxx: Optimise atu_get

Lookup in the ATU can be performed starting from a given MAC
address. This is faster than starting with the first possible MAC
address and iterating all entries.

Entries are returned in numeric order. So if the MAC address returned
is bigger than what we are searching for, we know it is not in the
ATU.

Using the benchmark provided by Volodymyr Bendiuga
<volodymyr.bendiuga@gmail.com>,

https://www.spinics.net/lists/netdev/msg411550.html



on an Marvell Armada 370 RD, the test to add a number of static fdb
entries went from 1.616531 seconds to 0.312052 seconds.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>