bgmac: initialize the DMA controller of core rev >= 4

The DMA controller used in the device supported by GMAC with core rev
>= 4 has some new options which are now set to the default values used
in the Broadcom SDK.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>