net: bcmgenet: enable NETIF_F_HIGHDMA flag

This commit configures the DMA masks for the GENET driver and
sets the NETIF_F_HIGHDMA flag to report support of the feature.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>