net: lwm2m: default LWM2M_LOCAL_PORT to 0 (random)

Due to a bug where LWM2M_LOCAL_PORT was not being honored, all
outgoing traffic from the LwM2M client was coming from a random
port determined during initialization.

Now that this bug bas been fixed, let's default the client to the
behavior that most users are expecting, and let new users customize
the outgoing port if needed (which should be rarely).

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>