Bluetooth: SMP: Fix peripheral security request with authenticated keys

The call to bt_conn_security will send a SMP security request for
peripheral, and for central it will initiate LL encryption.
A call to bt_conn_security with no IO capabilities but authenticated
keys has been distributed, would succeed on central side, but fail on
peripheral side with error code -22.

The keys could have been either:
 - Preprogrammed
 - IO capabilities may have changed.
 - OOB bonding may been used.

Fix so that Peripheral can send a security request if the bond
information is already established.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>