HID: fix memleaking of collection

hid_free_device() doesn't free device->collection (but it does
free device->rdesc and device itself). This imposes memory leak.
Fix it.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index b8cf50f..49f18f5 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -543,6 +543,7 @@
 	}
 
 	kfree(device->rdesc);
+	kfree(device->collection);
 	kfree(device);
 }
 EXPORT_SYMBOL_GPL(hid_free_device);