Bluetooth: controller: fixing issue re. assert on overlapping conn upd proc

If a connection update (from central) overlaps a peripheral initiated
conneection param request, then central rejects peripheral request
and continues central procedure. This lead to an assert in peripheral
On instant there would not be an rx_node for notification, as this
would have been discarded by the reception of the REJECT.

This fix ensures that once an rx_node has been 'accepted' for retention
it will not be discarded/overwritten by future rx_nodes

The test will trigger the assert without the fix in.

Signed-off-by: Erik Brockhoff <erbr@oticon.com>