HID: playstation: Document spinlock_t usage

Document the usage of 'lock' member in struct ps_device and silent
checkpatch.pl complaint:

  CHECK: spinlock_t definition without comment

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Benjamin Tissoires <bentiss@kernel.org>
Tested-by: Benjamin Tissoires <bentiss@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.com>

Author: cristicc

drivers/hid/hid-playstation.c

@@ -38,7 +38,7 @@ enum PS_TYPE {
 struct ps_device {
 	struct list_head list;
 	struct hid_device *hdev;
-	spinlock_t lock;
+	spinlock_t lock; /* Sync between event handler and workqueue */
 
 	u32 player_id;