Bluetooth: Fix using hci_conn_get() for hci_conn pointers

Wherever we keep hci_conn pointers around we should be using
hci_conn_get/put to ensure that they stay valid. This patch fixes
all places violating against the principle currently.

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