ARM: dmabounce: fix map_single() error return value

When map_single() is unable to obtain a safe buffer, we must return
the dma_addr_t error value, which is ~0 rather than 0.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/arm/common/dmabounce.c b/arch/arm/common/dmabounce.c
index e568163..841df7d2 100644
--- a/arch/arm/common/dmabounce.c
+++ b/arch/arm/common/dmabounce.c
@@ -255,7 +255,7 @@
 		if (buf == 0) {
 			dev_err(dev, "%s: unable to map unsafe buffer %p!\n",
 			       __func__, ptr);
-			return 0;
+			return ~0;
 		}
 
 		dev_dbg(dev,