net: NULL the old xps map entries when freeing them

In __netif_set_xps_queue, old map entries from the old dev_maps are
freed but their corresponding entry in the old dev_maps aren't NULLed.
Fix this.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>