firewire: fix compiler warnings on 64bit

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Kristian Høgsberg <krh@redhat.com>
diff --git a/drivers/firewire/fw-device.c b/drivers/firewire/fw-device.c
index 415e9ab..a834221 100644
--- a/drivers/firewire/fw-device.c
+++ b/drivers/firewire/fw-device.c
@@ -240,7 +240,7 @@
 	struct fw_unit *unit = fw_unit(dev);
 
 	return snprintf(buf, PAGE_SIZE, "%d\n",
-			unit->directory - device->config_rom);
+			(int)(unit->directory - device->config_rom));
 }
 
 static struct device_attribute rom_index_attribute = {