net: lwm2m: Prevent client restart on bootstrap finish retransmission

Ignore bootstrap 'transition done' state transition
(ENGINE_BOOTSTRAP_TRANS_DONE) from an invalid state. The client must be
in state ENGINE_BOOTSTRAP_REG_DONE to process a bootstrap finish command
received from the server.

The LwM2M client does not implement CoAP deduplication (rfc 7252,
section 4.5) and retransmissions of the bootstrap finish command `/bs`
would restart the registration client with undesired side effects.

Signed-off-by: Marc Lasch <marc.lasch@husqvarnagroup.com>