Bluetooth: Fix resetting remote authentication requirement after pairing

When a new hci_conn object is created the remote SSP authentication
requirement is set to the invalid value 0xff to indicate that it is
unknown. Once pairing completes however the code was leaving it as-is.
In case a new pairing happens over the same connection it is important
that we reset the value back to unknown so that the pairing code doesn't
make false assumptions about the requirements.

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