ethoc: fix buffer address mapping

The pointer address in buffer descriptors is physical address. The
pointer that processor used to access packet is virtual address.

Though the higher bits of pointer address used by the MAC may be
truncated to zero in special case, it is not always true in larger
designs.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: David S. Miller <davem@davemloft.net>