nfc: NULL vs zero in nci_activate_target()

This is a pointer so it should be NULL instead of zero.  Sparse
complains about this stuff:
net/nfc/nci/core.c:447:37: warning: Using plain integer as NULL pointer
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/nfc/nci/core.c

@@ -444,7 +444,7 @@ static int nci_activate_target(struct nfc_dev *nfc_dev, __u32 target_idx,
 {
 	struct nci_dev *ndev = nfc_get_drvdata(nfc_dev);
 	struct nci_rf_discover_select_param param;
-	struct nfc_target *target = 0;
+	struct nfc_target *target = NULL;
 	int i;
 	int rc = 0;