amd-xgbe: Set DMA mask based on hardware register value

The hardware supplies a value that indicates the DMA range that it
is capable of using. Use this value rather than hard-coding it in
the driver.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>