net: mhi: remove pointless conditional before kfree_skb()

It already has null pointer check in kfree_skb(),
remove pointless pointer check before kfree_skb().

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>