ethoc: Write bus addresses to registers

The ethoc driver should be writing bus addresses to the ethoc registers, not
virtual addresses.  This patch adds an array to store the virtual addresses
in and references that array when manipulating the contents of the buffer
descriptors.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: David S. Miller <davem@davemloft.net>