net: lwm2m: honor CONFIG_LWM2M_LOCAL_PORT when starting client

Currently, CONFIG_LWM2M_LOCAL_PORT is never used when setting up
the LwM2M client.  Let's set the port of the local address using
CONFIG_LWM2M_LOCAL_PORT, so that the client can bind to it.

NOTE: A setting of 0 will use a random port.

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