net: lwm2m: fix all return values from resource callbacks

Previously, post_write and execute callbacks returned 1 when handled
and 0 for error condition.  However, this wasn't detailed enough and
the engine can't propagate any sort of error back to users -- so it
doesn't even check the return values in many cases!

Let's adjust the resource callback functions of all objects and the
lwm2m_client sample to return 0 for success or a valid error code.

Signed-off-by: Michael Scott <michael.scott@linaro.org>