bluetooth: controller: Added missing variable initializations

In ull.c ll_rx_get, a configuration without CONFIG_BT_CONN would
sometimes return an uninitialized node_rx.
In ull_scan.c, the scanning channel was not initialized to 0 (37). This
would cause new scanning to start at random index (0,1,2).

Signed-off-by: Morten Priess <mtpr@oticon.com>