xfrm: fix the if_id check in changelink

if_id will be always 0, because it was not yet initialized.

Fixes: 8dce4391 ("xfrm: interface with if_id 0 should return error")
Reported-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>