firewire: move fw_device reference counting from drivers to core

fw_unit device drivers invariably need to talk to the fw_unit's parent
(an fw_device) and grandparent (an fw_card).  firewire-core already
maintains an fw_card reference for the entire lifetime of an fw_device.
Likewise, let firewire-core maintain an fw_device reference for the
entire lifetime of an fw_unit so that fw_unit drivers don't have to.

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 f3b890d..4c6c7d8 100644
--- a/drivers/firewire/core-device.c
+++ b/drivers/firewire/core-device.c
@@ -641,6 +641,7 @@
 {
 	struct fw_unit *unit = fw_unit(dev);
 
+	fw_device_put(fw_parent_device(unit));
 	kfree(unit);
 }
 
@@ -692,6 +693,7 @@
 		if (device_register(&unit->device) < 0)
 			goto skip_unit;
 
+		fw_device_get(device);
 		continue;
 
 	skip_unit: