we have an ASA 5540 that require LAN port for failover. And the only available interface left is the management port. How to config management interface to an ethernet interface?
You can disable the management-only mode on that interface to make it as regualr routable port and use for any other purpose including LAN base failover purposes.
About management interface - 5510 but generally applies to management0/0 itself including 5540
configuring LAN base failover