Bluetooth: SSP: clear pairing flag if ssp pairing completed

When the ssp pairing event is notified, clear the pairing
flag.

It is used to support case that the authentication is
started by peer, the link will not be encrypted after
the pairing is completed without any err.
If the local device want to encrypt the link, it could
call `bt_conn_set_security` to start encrypt the link
after the pairing complete callback triggered.

In original implementation, the `bt_conn_set_security`
cannot be called if the authentication has been started.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>