[nrf fromtree] net: lib: coap_client: Fix timeout for separate response

When waiting for response after receiving the empty Ack, client
actually used way too timeout.
CoAP timeout only holds the timeout value in ms. t0 is the starting time.

Signed-off-by: Seppo Takalo <[email protected]>
(cherry picked from commit 6c16966)

Author: SeppoTakalo

subsys/net/lib/coap/coap_client.c

@@ -793,7 +793,7 @@ static int handle_response(struct coap_client *client, const struct coap_packet
 			return 0;
 		}
 		internal_req->pending.t0 = k_uptime_get();
-		internal_req->pending.timeout = internal_req->pending.t0 + COAP_SEPARATE_TIMEOUT;
+		internal_req->pending.timeout = COAP_SEPARATE_TIMEOUT;
 		internal_req->pending.retries = 0;
 		return 1;
 	}