dwmmc: Reset the dma controller at attach

If the bootloader enabled DMA we need to fully reset the DMA controller
otherwise we might have some stale data in it that provoke weird
behavior.

MFC after:	1 week

sys/dev/mmc/host/dwmmc.c

@@ -611,6 +611,7 @@ dwmmc_attach(device_t dev)
 	}
 
 	if (!sc->use_pio) {
+		dma_stop(sc);
 		if (dma_setup(sc))
 			return (ENXIO);