Bluetooth: Don't modify sec_level if auth failed

If authentication fails the security level should stay as it was set
before the process has started. Setting BT_SECURITY_LOW can hide real
security level on a link eg. having BT_SECURITY_MEDIUM on the link,
re-authenticate with failure to get BT_SECURITY_HIGH, as  a result we
get BT_SECURITY_LOW on the link while the real security is still medium.

Signed-off-by: Waldemar Rymarkiewicz <waldemar.rymarkiewicz@tieto.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>