mctp: remove unnecessary check before calling kfree_skb()

The skb will be checked inside kfree_skb(), so remove the
outside check.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20211130031243.768823-1-yangyingliang@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>