net: tcp: Do not handle packet re-transmission in TCP ACK

Zephyr doesn't have luxury to create re-transmit timer per packet. So
it has different methods to handle packets in queue. But re-sending
packets on valid ack messages causing issues.

E.g. A TCP node sent two packets (packet-1, packet-2). Peer replied
two ACKs (ACK-1 and ACK-2), and these two ACK's are at rx_thread
queue. Now ACK-1 is handled and reference of packet-1 is freed
from sent list. Then if condiftion (valid ACK and connection
state is ESTABLISHED) notices that, sent list is not empty.
Restart the timer, modify sent flag and resend packets in a
list. Here packet-2 is sent again, even though ACK-2 is already
received. Situation is worse if there are more packets in the
list.

So only start the re-transmit timer in-case queue is not empty. It
allows rx_thread to handle all incoming packets (in this e.g ACKs).
When the re-trasmit timer expires, it sends the packets which
are left in queue.

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>