xfrm interface: don't take extra reference to netdev

I don't see any reason to do this. Maybe needed before
commit 56c5ee1a

 ("xfrm interface: fix memory leak on creation").

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>