Bluetooth: Controller: Use hci.h for num complete

Use Zephyr's include/bluetooth/hci.h for the HCI event
Number of Completed Packets.
This is the third commit in a series that transitions from
the event structures in hci.c to the ones in hci.h.

Jira: ZEP-726

Change-Id: I179bceb2f9dc07d7a1a524c3a80f0886f712ec2b
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>