net: tcp: Fix packet processing result reporting

TCP implementation abused the packet processing result reporting
mechanism, by reporting NET_DROP for every packet other than data
packets (which were passed to the application). This simplified the TCP
implementation a bit, as it didn't need to deal with packet releasing.
While technically it worked, it gave incorrect results when it comes to
packet statistics, as a lot of valid TCP packets were counted as
dropped.

This commit fixes this. Each packet that was properly processed by the
TCP stack is not either released at the TCP level, or passed to the
application. In both cases, NET_OK is returned to the network core,
indicating the packet has been properly processed and consumed.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>