Bluetooth: Add hci_conn_lookup_type() helper function

Some drivers require knowledge of what connection handle is assigned
to what connection link type (ACL or SCO/eSCO). Instead of having each
driver implement connection tracking, provide a simple helper function
for lookup of the link type.

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