drivers/net/netxen: fix sparse warnings: use NULL pointer instead of plain integer

Fix this sparse warnings:

  drivers/net/netxen/netxen_nic_hw.c:1462:18: warning: Using plain integer as NULL pointer
  drivers/net/netxen/netxen_nic_hw.c:1536:18: warning: Using plain integer as NULL pointer

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>