Bluetooth: Controller: Fix ull_hdr_get_cb for Peripheral role

Fix ull_hdr_get_cb() for advanced scheduling to return
ticks_slot for Peripheral role.

Relates to commit d6c3e04e ("Bluetooth: Controller:
Central maximum data PDU size time spacing").

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