net: dsa: mv88e6xxx: Add devlink param for ATU hash algorithm.

Some of the marvell switches have bits controlling the hash algorithm
the ATU uses for MAC addresses. In some industrial settings, where all
the devices are from the same manufacture, and hence use the same OUI,
the default hashing algorithm is not optimal. Allow the other
algorithms to be selected via devlink.

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