net: lwm2m: fix logic for lwm2m_next_engine_obj_inst()

The object instance list isn't sorted by object instance id.  Let's
simplify this and fix the logic in lwm2m_next_engine_obj_inst() to make
sure that we always get the NEXT object instance by value of
obj_inst_id, not just the next object instance in the list.

NOTE: This change removes the "last" object instance pointer from the
parameters of lwm2m_next_engine_obj_inst().  Some of the logic to return
a NULL value for the end of the list has to be moved back into
do_read_op().

Signed-off-by: Michael Scott <mike@foundries.io>