net: lwm2m: Improve TLV content encoder/decoder error handling

It's now possible to return an error code instead of 0 where
appropriate.

Make `buf_read()` return -ENODATA instead of -ENOMEM if there's not
enough data in the packet to serve the request, which is more meaningful
for this scenario.

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