Fix following gcc warning:

drivers/net/ipvlan/ipvlan_main.c:543:12: warning:
 comparison is always false due to limited range of data type [-Wtype-limits]

'mode' is a u16 variable, IPVLAN_MODE_L2 is zero,
the comparison is always false

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>