net: tcp: fix spurious TCP retries

Spurious TCP retries were observed using Wireshark while continuously
sending TCP packets at an interval faster than the initial RTO.

If the send list is empty and CONFIG_NET_TCP_TIME_WAIT_DELAY is used,
the retry timer will not be correctly stopped when receiving a valid
ACK. As a consequence, the timer might be running when a new packet is
queued, but the logics in net_tcp_queue_data() will not restart the
timer as it is already running. This will make the retry timer to expire
prematurely, potentially while sending packets.

The nested condition is merged into a single condition, allowing the
final else clause to be reached when a valid ACK is received.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>