Bluetooth: Fix IRK lookup when tfm_aes is not available

If the AES crypto has not been initialized properly we should cleanly
return from the hci_find_irk_by_rpa() function. Right now this will not
happen in practice, but once (in subsequent patches) SMP init is moved
to after the HCI init procedure it is possible that the pointer is NULL.

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