Bluetooth: Fix handling of getsockname() for HCI sockets

The hci_dev check is not protected and so move it into the socket lock. In
addition return the HCI channel identifier instead of always 0 channel.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>