net: lwm2m: Fix possible out-of-bound access when creating FW object

obj_inst_id should not be used directly to index the instance array, as
the instance ID is not tightly bound to the maximum instance count and
can exceed this value, causing out-of-bound access.

Therefore, perform some extra validation when choosing the array index
for the object instance to make sure we stay in the array bounds, or
return an error if there's no more room for more object instances, in a
similar way it's done for Security object.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>