[SCSI] qla2xxx: use NULL instead of 0

Use NULL instead of 0 for pointers (sparse warning):
drivers/scsi/qla2xxx/qla_attr.c:393:4: warning: Using plain integer as NULL pointer

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>