net: ftgmac100: fix ring allocation unwind on open failure

In the Linux kernel, the following vulnerability has been resolved: net: ftgmac100: fix ring allocation unwind on open failure ftgmac100_alloc_rings() allocates rx_skbs, tx_skbs, rxdes, txdes, and rx_scratch in stages. On intermediate failures it returned -ENOMEM directly, leaking resources allocated earlier in the function. Rework the failure path to use staged local unwind labels and free allocated resources in reverse order before returning -ENOMEM. This matches common netdev allocation cleanup style.