net: mana: Fix possible double free in error handling path

In the Linux kernel, the following vulnerability has been resolved: net: mana: Fix possible double free in error handling path When auxiliary_device_add() returns error and then calls auxiliary_device_uninit(), callback function adev_release calls kfree(madev). We shouldn't call kfree(madev) again in the error handling path. Set 'madev' to NULL.