net: hns3: fix vf id check issue when add flow director rule

When add flow director fule for vf, the vf id is used as array
subscript before valid checking, which may cause memory overflow.

Fixes: dd74f815

 ("net: hns3: Add support for rule add/delete for flow director")
Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>