tun: fix inability to set offloads after disabling them via ethtool

Added missing logic in tun driver, which prevents apps to set
offloads using tun ioctl, if offloads were previously disabled via ethtool

Signed-off-by: Yaroslav Isakov <yaroslav.isakov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>