Bluetooth: Controller: Fix pdu_free_sem_give assertion under ZLI use

Fix assertion due to multiple mayfly_enqueue calls used
under ZLI when pdu_free_sem_give is invoked from the LLL.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
(cherry picked from commit ae8e7f4c)