Eliminate hard-coded address of Ethernet transfer buffer on at91rm9200

Patch by Anders Larsen, 07 Oct 2005

The Atmel errata #11 states that the transfer buffer descriptor
table must be aligned on a 16-word boundary. As it turned out, this
is insufficient - it seems the table must be aligned on a boundary
at least as large as the table itself (in Linux this is not an
issue - the table is aligned on a PAGE_SIZE (4096) boundary).