net: airoha: Do not read uninitialized fragment address in airoha_dev_xmit()

In the Linux kernel, the following vulnerability has been resolved: net: airoha: Do not read uninitialized fragment address in airoha_dev_xmit() The transmit loop in airoha_dev_xmit() reads fragment address and length during its final iteration, when the loop index equals skb_shinfo(skb)->nr_frags, at which point the fragment data is uninitialized. While these values are never consumed, the read itself is unsafe and may trigger a page fault. Fix this by avoiding the fragment read on the last iteration. Additionally, move the skb pointer from the first to the last used packet descriptor, so that airoha_qdma_tx_napi_poll() defers freeing the skb until the final descriptor is processed.