dma: Fix stupid typo/thinko

Hi hard a brain fart when coding that function, it will
fail to "set" the memory beyond the first 512 bytes. This
is in turn causing guest crashes in ibmveth (spapr_llan.c
on the qemu side) due to the receive queue not being
properly initialized.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
