net: calxedaxgmac: determine number of address filters at runtime

Highbank and Midway xgmac h/w have different number of MAC address filter
registers with 7 and 31, respectively. Highbank has been wrong, so fix it
and detect the number of filter registers at run-time. Unfortunately,
the version register is the same on both SOCs, so simply test if write to
the last filter register will take a value. It always reads as 0 if not.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>