gre6: Move the setting of dev->iflink into the ndo_init functions.

[ Upstream commit f03eb128e3f4276f46442d14f3b8f864f3775821 ]

Otherwise it gets overwritten by register_netdev().

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>