netfilter: xt_TEE: add missing code to get interface index in checkentry.

checkentry(tee_tg_check) should initialize priv->oif from dev if possible.
But only netdevice notifier handler can set that.
Hence priv->oif is always -1 until notifier handler is called.

Fixes: 9e2f6c5d ("netfilter: Rework xt_TEE netdevice notifier")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>