net: tcp: Fix FIN with data handling

In case FIN packet included data bytes, Zephyr would ignore the
data. It wasn't passed to the application, and it wasn't considered
when bumping the ACK counter. This ended out in connection timing out,
instead of being properly terminated.

Fix this, by refactoring FIN processing in TCP_ESTABLISHED state.
Instead of handling FIN/FIN,ACK/FIN,ACK,PSH cases separately, have a
common handler when FIN flag is present, and when ACK flag is present
along with FIN. When FIN is present, take any potentially incoming data
into account.

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