Bluetooth: controller: split: Dont use continuous directed adv in nRF51

In nRF51 which uses CONFIG_BT_CTLR_LOW_LAT, the advertising
PDU tend to get aborted in directed advertising at event slot
durations.

Dont not use continuous directed advertising event in nRF51
where CONFIG_BT_CTLR_LOW_LAT scheduling design alternative
is used. Instead close the event after each triplet of PDU
has been tx-ed.

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