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>