bluetooth: att: Fix L2CAP channel section order

The L2CAP channel section is sorted lexicographically. Make sure
that ATT fixed channel will be placed as the last one to ensure
that SMP channel is properly initialized before bt_att_connected
tries to send security request.

Fixes #45820

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>