v4l2_ctrl_handler_free() uses hdl->lock, which in ov5640 driver is set
to sensor's own sensor->lock. In ov5640_remove(), the driver destroys the
sensor->lock first, and then calls v4l2_ctrl_handler_free(), resulting
in the use of the destroyed mutex.
Fix this by calling moving the mutex_destroy() to the end of the cleanup
sequence, as there's no need to destroy the mutex as early as possible.