net: tcp: Reschedule FIN timer when entering FIN state

The FIN timer was not set when we entered the FIN_WAIT_1 state.
This could cause issues if we did not receive proper packets
from peer. With this fix, the connection is always terminated
even if peer does not respond.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>