net: Remove duplicated midx check against 0

Check midx against 0 is always equal to check midx against sk_bound_dev_if
when sk_bound_dev_if is known not equal to 0 in these case.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>