logsys: Fix signedness of get_unblocked

_logsys_config_subsys_get_unlocked returns signed int so use it for
iterator.

Also correctly use signed int for result in _logsys_config_subsys_get
function.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>