Bluetooth: Don't trigger disconnect timeout for security mode 3 pairing

A remote device in security mode 3 that tries to connect will require
the pairing during the connection setup phase. The disconnect timeout
is now triggered within 10 milliseconds and causes the pairing to fail.

If a connection is not fully established and a PIN code request is
received, don't trigger the disconnect timeout. The either successful
or failing connection complete event will make sure that the timeout
is triggered at the right time.

The biggest problem with security mode 3 is that many Bluetooth 2.0
device and before use a temporary security mode 3 for dedicated
bonding.

Based on a report by Johan Hedberg <johan.hedberg@nokia.com>

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Tested-by: Johan Hedberg <johan.hedberg@nokia.com>