staging: wfx: drop useless link_map_cache

Driver does not use link_map_cache anymore. So we can drop it. In add,
we do not have to keep this counter in sync with skb_queue item, so we
can drop explicit spin_locks.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200401110405.80282-16-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>