Bluetooth: zero kpp input for key generation

When generating new ECDH keys with kpp, the shared secret input needs to
be set to NULL. Fix this by including kpp_request_set_input call.

Fixes: 58771c1c ("Bluetooth: convert smp and selftest to crypto kpp
API")
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>