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>

drivers/usb/musb/cppi_dma.c

@@ -1204,7 +1204,7 @@ irqreturn_t cppi_interrupt(int irq, void *dev_id)
 		 */
 		if (NULL == bd) {
 			DBG(1, "null BD\n");
-			tx_ram->tx_complete = 0;
+			musb_writel(&tx_ram->tx_complete, 0, 0);
 			continue;
 		}