net: tcp: Increase default NET_TCP_TIME_WAIT_DELAY value

The current default of 250ms appears to be too low in case connection
teardown takes place in lossy networks - in case of FIN packet
retransmission, the connection on the Zephyr side could have already
been dismissed due to low TIME_WAIT state delay, resulting in ICMP
Destination Unreachable replies.

Increase the default value to 1500ms - this is still pretty low, but at
least gives the peer some time to retransmit the FIN packet.

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