ipv6: remove a useless NULL check

In dev_forward_change(), it is useless to check if idev->dev
is NULL, it is always non-NULL here.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>