Bluetooth: Allow change security level on ATT_CID in slave role

Kernel supports SMP Security Request so don't block increasing security
when we are slave.

Signed-off-by: Marcin Kraglak <marcin.kraglak@tieto.com>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable@vger.kernel.org