USB: musb: dereferencing an iomem pointer

"tx_ram" points to io memory.  We can't dereference it directly.  Sparse
complains about this: "drivers/usb/musb/cppi_dma.c:1205:25: warning:
dereference of noderef expression"

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/musb/cppi_dma.c b/drivers/usb/musb/cppi_dma.c
index eb9161a..ab434fb 100644
--- a/drivers/usb/musb/cppi_dma.c
+++ b/drivers/usb/musb/cppi_dma.c
@@ -1204,7 +1204,7 @@
 		 */
 		if (NULL == bd) {
 			DBG(1, "null BD\n");
-			tx_ram->tx_complete = 0;
+			musb_writel(&tx_ram->tx_complete, 0, 0);
 			continue;
 		}