net: tcp: don't assume TCP_FIN buffers are NET_DROP

Since we default to a return value of NET_DROP, we
can remove the automatic NET_DROP in the TCP_FIN block.

The return value will be set to NET_OK by packet_received()
if appropriate data is found which needs to be sent to
the callback.

Change-Id: Ib2634ba34440ca7053a4e98bf80f12cf6fbbd361
Signed-off-by: Michael Scott <michael.scott@linaro.org>