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

dma-helpers.c

@@ -24,8 +24,8 @@ static void do_dma_memory_set(dma_addr_t addr, uint8_t c, dma_addr_t len)
     while (len > 0) {
         l = len < FILLBUF_SIZE ? len : FILLBUF_SIZE;
         cpu_physical_memory_rw(addr, fillbuf, l, true);
-        len -= len;
-        addr += len;
+        len -= l;
+        addr += l;
     }
 }