Bluetooth: 6lowpan: Increase the connection timeout value

Use the default connection timeout value defined in l2cap.h because
the current timeout was too short and most of the time the connection
attempts timed out.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable@vger.kernel.org # 3.17.x