staging: vt6656: Add comment for locks

This patch fixes the checkpatch.pl warnings like:
- CHECK: spinlock_t definition without comment
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/c6a5ef8e7704b488c54145b09ac44bd4880c13b4.1645477326.git.philipp.g.hortmann@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vt6656/device.h

@@ -268,8 +268,8 @@ struct vnt_private {
 	u32 rx_buf_sz;
 	int mc_list_count;
 
-	spinlock_t lock;
-	struct mutex usb_lock;
+	spinlock_t lock;		/* prepare tx USB URB */
+	struct mutex usb_lock;		/* USB control messages */
 
 	unsigned long flags;