Bluetooth: Remove hci_conn_hold/drop from hci_chan

We can't have hci_chan contribute to the "active" reference counting of
the hci_conn since otherwise the connection would never get dropped when
there are no more users (since hci_chan would be counted as a user).
This patch removes hold() when creating the hci_chan and drop() when
destroying it.

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