staging: wfx: relocate wfx_skb_dtor() prior its callers

In a next commit, we would like to mark wfx_skb_dtor as static and stop
to declare it in data_tx.h.

Relocate wfx_skb_dtor() prior its callers to avoid compile error.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200401110405.80282-28-Jerome.Pouiller@silabs.com


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