Bluetooth: Reject pairing requests when in non-pairable mode

This patch adds the necessary logic to act accordingly when the
HCI_PAIRABLE flag is not set. In that case PIN code replies as well as
Secure Simple Pairing requests without a NoBonding requirement need to
be rejected.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>