parisc/parport_gsc: Use NULL to avoid sparse warning

Signed-off-by: Helge Deller <deller@gmx.de>

drivers/parport/parport_gsc.c

@@ -256,7 +256,7 @@ struct parport *parport_gsc_probe_port(unsigned long base,
 	}
 	priv->ctr = 0xc;
 	priv->ctr_writable = 0xff;
-	priv->dma_buf = 0;
+	priv->dma_buf = NULL;
 	priv->dma_handle = 0;
 	p->base = base;
 	p->base_hi = base_hi;