macb: support DMA bus widths > 32 bits

Some GEM implementations may support DMA bus widths up to 128 bits.  We
can get the maximum supported DMA bus width from the design
configuration register so use that to program the device up.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>