firewire: add parent-of-unit accessor

Retrieval of an fw_unit's parent is a common pattern in high-level code.
Wrap it up as device = fw_parent_device(unit).

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
diff --git a/drivers/firewire/core-device.c b/drivers/firewire/core-device.c
index 65d84dd..3f4e646 100644
--- a/drivers/firewire/core-device.c
+++ b/drivers/firewire/core-device.c
@@ -93,7 +93,7 @@
 	if (!is_fw_unit(dev))
 		return 0;
 
-	device = fw_device(unit->device.parent);
+	device = fw_parent_device(unit);
 	id = container_of(drv, struct fw_driver, driver)->id_table;
 
 	for (; id->match_flags != 0; id++) {
@@ -114,7 +114,7 @@
 
 static int get_modalias(struct fw_unit *unit, char *buffer, size_t buffer_size)
 {
-	struct fw_device *device = fw_device(unit->device.parent);
+	struct fw_device *device = fw_parent_device(unit);
 	struct fw_csr_iterator ci;
 
 	int key, value;