atm: nicstar: use NULL instead of 0 for pointer

sparse says:

drivers/atm/nicstar.c:642:27: warning:
 Using plain integer as NULL pointer
drivers/atm/nicstar.c:644:27:
 warning: Using plain integer as NULL pointer
drivers/atm/nicstar.c:982:51:
 warning: Using plain integer as NULL pointer
drivers/atm/nicstar.c:996:51:
 warning: Using plain integer as NULL pointer

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Acked-by: Chas Williams <chas@cmf.nrl.navy.mil>
Signed-off-by: David S. Miller <davem@davemloft.net>