Bluetooth: Fix rejecting pairing in case of insufficient capabilities

If we need an MITM protected connection but the local and remote IO
capabilities cannot provide it we should reject the pairing attempt in
the appropriate way. This patch adds the missing checks for such a
situation to the smp_cmd_pairing_req() and smp_cmd_pairing_rsp()
functions.

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