net: lwm2m: fixed "LwM2M message is invalid"

Do not retransmit a message that just has been send successfully.

This case can in particular happen quite frequently if the modem
connection/throughput is quite bad and so there is a high latency.

A message that has to be acknowledged is scheduled for retransmission.
Before retransmission a ack for this message is received that will
cause a reset of the original message. In this case you see a
"LwM2M message is invalid" error message.

Signed-off-by: Andreas Chmielewski <andreas.chmielewski@grandcentrix.net>