Bluetooth: Fix hci_conn reference counting with hci_chan

The hci_chan_del() function was doing a hci_conn_drop() but there was no
matching hci_conn_hold() in the hci_chan_create() function. Furthermore,
as the hci_chan struct holds a pointer to the hci_conn there should be
proper use of hci_conn_get/put. This patch fixes both issues so that
hci_chan does correct reference counting of the hci_conn object.

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