net: coap: Fix coap client observe out-of-order messages

coap_response_received returned NULL if the observe option was out of
order, however it makes more sense to return the coap_reply handler
without actually calling it.

Additionally the reorder check has been modified to partially match
the RFC.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>