Bluetooth: Controller: Refactor HCI files

As part of the effort to consolidate the BLE Controller's
HCI layer, the following files have been renamed:

* hci.h -> hci_internal.h: contains the HCI API to be used
by the driver
* main.c -> hci_driver.c: Implement bt_driver and includes
initialization and glue code

Jira: ZEP-726

Change-Id: Ica8b3e114da42a766a1b14ce59558cacd899a1a7
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>