Bluetooth: Move hci_get_cmd_complete() to hci_event.c

To make the hci_req_run_skb() API consistent with hci_cmd_sync_ev()
the callback should receive the cmd_complete parameters in the 'normal'
case and the full HCI event if a special event was expected. This patch
moves the hci_get_cmd_complete() function from hci_core.c to hci_event.c
where it's used to strip the skb from the needed headers before passing
it on to the callback.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>