Bluetooth: Controller: Avoid division by 0 with BT_CTLR_THROUGHPUT

With very fast HCI interfaces (for ex. in the simulated nrf5340)
the tx_rate calculation can do a division by 0
(if to packets are enqueued in the same 32KHz clock period).
Let's avoid it.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>