Bluteooth: Reject SMP bonding if HCI_PAIRABLE is not set

If the remote device tries to initiate bonding with us and we don't have
HCI_PAIRABLE set we should just flat out reject the request. This brings
SMP in line with how the flag is used for BR/EDR SSP.

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