liquidio: Using NULL instead of plain integer

Fixes the following sparse warnings:

drivers/net/ethernet/cavium/liquidio/lio_main.c:3068:23: warning:
 Using plain integer as NULL pointer
drivers/net/ethernet/cavium/liquidio/lio_main.c:2909:23: warning:
 Using plain integer as NULL pointer
drivers/net/ethernet/cavium/liquidio/cn23xx_vf_device.c:385:27: warning:
 Using plain integer as NULL pointer

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