Bluetooth: Controller: Fix missing recv fifo reset

Fix missing recv fifo reset on HCI reset. This fix handles a
scenario where in Rx Prio thread has enqueued a node rx, Tx
thread handles HCI Reset Command, and Rx thread wakes up
from call to k_fifo_get to handle invalid node rx. The
changes here ensure Rx thread does not get any invalid node
rx post HCI Reset Command handled in Tx thread.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>