net: hsr: remove redundant null check

Because kfree_skb already checked NULL skb parameter,
so the additional checks are unnecessary, just remove them.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>