Disable ATAPI DMA as it's once again broken in that it causes data

corruption with the on-board AcerLabs M5229 controllers. While at
it, remove the pointless "nothing to autoload yet." message.

MFC after:	3 days

sys/boot/sparc64/loader/main.c

@@ -261,7 +261,7 @@ static int
 sparc64_autoload(void)
 {
 
-	printf("nothing to autoload yet.\n");
+	setenv("hw.ata.atapi_dma", "0", 0);
 	return (0);
 }