Bluetooth: controller: Fix multiple master role event scheduling

When the controller is connecting to multiple connectable
advertisers, the events are scheduled consecutively avoiding
overlapping events. Calculation of the window offset did not
consider the preparation time before the event, causing the
new master role connection event to overlap with previous
event. This is now fixed by including the preparation time
in the used window offset from the end of connect_ind PDU
transmitted.

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