Bluetooth: ISO: handle bound CIS cleanup via hci_conn

Calling hci_conn_del in __iso_sock_close is invalid. It needs
hdev->lock, but it cannot be acquired there due to lock ordering.

Fix this by doing cleanup via hci_conn_drop.

Return hci_conn with refcount 1 from hci_bind_cis and hci_connect_cis,
so that the iso_conn always holds one reference.  This also fixes
refcounting when error handling.

Since hci_conn_abort shall handle termination of connections in any
state properly, we can handle BT_CONNECT socket state in the same way as
BT_CONNECTED.

Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>