powerpc/scom: Change scom_read() and scom_write() to return errors

scom_read() now returns the read value via a pointer argument and
both functions return an int error code

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/sysdev/scom.c

@@ -137,8 +137,7 @@ static int scom_val_get(void *data, u64 *val)
 	if (!scom_map_ok(ent->map))
 		return -EFAULT;
 
-	*val = scom_read(ent->map, 0);
-	return 0;
+	return scom_read(ent->map, 0, val);
 }
 DEFINE_SIMPLE_ATTRIBUTE(scom_val_fops, scom_val_get, scom_val_set,
 			"0x%llx\n");