Bluetooth: Improve *_get() functions to return the object type

It's natural to have *_get() functions that increment the reference
count of an object to return the object type itself. This way it's
simple to make a copy of the object pointer and increase the reference
count in a single step. This patch updates two such get() functions,
namely hci_conn_get() and l2cap_conn_get(), and updates the users to
take advantage of the new API.

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