accel/amdxdna: Fix NULL pointer dereference of mgmt_chann

In the Linux kernel, the following vulnerability has been resolved: accel/amdxdna: Fix NULL pointer dereference of mgmt_chann mgmt_chann may be set to NULL if the firmware returns an unexpected error in aie2_send_mgmt_msg_wait(). This can later lead to a NULL pointer dereference in aie2_hw_stop(). Fix this by introducing a dedicated helper to destroy mgmt_chann and by adding proper NULL checks before accessing it.