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 removes Nordic Semiconductor Vendor-Specific
commands, since they are currently unused and
manufacturer-specific.
This is the fourth commit in a series that transitions from
the structures in hci.c to the ones in hci.h.

Jira: ZEP-726

Change-Id: If0141072489777bfecc06e4aaa5f569898a5c449
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>