Bluetooth: Controller: Switch to Zephyr's hci.h for cmd handling

Continue using Zephyr's include/bluetooth/hci.h for HCI command
handling in more of the commands.
This commit ports the LE controller commands to hci.h.
This is the third commit in a series that transitions from
the structures in hci.c to the ones in hci.h.

Jira: ZEP-726

Change-Id: I3f11cca1da4aa6a20cce9706362818f8f6c87b0a
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>