Bluetooth: Controller: Fix RX_ENQUEUE_HOLD for LOW_LAT_ULL

Fix BT_CTLR_RX_ENQUEUE_HOLD implementation that stalled
generating the CONN_UPDATE and PHY_UPDATE when there were
no Rx data or Tx acknowledgements to be processed when
the controller was built with BT_CTLR_LOW_LAT_ULL.

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