powerpc/warp: Fix ISA_DMA_THRESHOLD default

If no device is passed to __dma_alloc_coherent, it defaults to using ISA_DMA_THRESHOLD
for the mask. This patch provides a reasonable default rather than 0.
Signed-off-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/44x/warp.c

@@ -43,7 +43,13 @@ static int __init warp_probe(void)
 {
 	unsigned long root = of_get_flat_dt_root();
 
-	return of_flat_dt_is_compatible(root, "pika,warp");
+	if (!of_flat_dt_is_compatible(root, "pika,warp"))
+		return 0;
+
+	/* For __dma_alloc_coherent */
+	ISA_DMA_THRESHOLD = ~0L;
+
+	return 1;
 }
 
 define_machine(warp) {