net: dsa: qca8k: Add check for skb_copy

Add check for the return value of skb_copy in order to avoid NULL pointer
dereference.

Fixes: 2cd54856 ("net: dsa: qca8k: add support for phy read/write with mgmt Ethernet")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/qca/qca8k-8xxx.c

@@ -588,6 +588,9 @@ qca8k_phy_eth_busy_wait(struct qca8k_mgmt_eth_data *mgmt_eth_data,
 	bool ack;
 	int ret;
 
+	if (!skb)
+		return -ENOMEM;
+
 	reinit_completion(&mgmt_eth_data->rw_done);
 
 	/* Increment seq_num and set it in the copy pkt */