Bluetooth: Add hdev_init callback for HCI channels

In order to make the mgmt command handling more generic we can't have a
direct call to mgmt_init_hdev() from mgmt_control(). This patch adds a
new callback to struct hci_mgmt_chan. And sets it to point to the
mgmt_init_hdev() function for the HCI_CHANNEL_CONTROL instance.

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