net: lwm2m: LwM2M RD client start and stop update

Added return code for for lwm2m_rd_client_start() & lwm2m_rd_client_stop().

lwm2m_rd_client_start() return -EINPROGRESS when start is in progress and
0 for success.

lwm2m_rd_client_stop() return -EPERM when context is unknown and
0 for success.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>