Bluetooth: Controller: Fix connection event busy check

Fix connection event busy check when peripheral role is
using minimal time reservation.

Peripheral and Central have a anchor point sync if the
Peripheral has successfully transmitted once to the Central.

Fixes commit cadef5a6 ("Bluetooth: Controller: Introduce
BT_CTLR_PERIPHERAL_RESERVE_MAX")'.

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