mmc: mxcmmc: fix falling back to PIO

When we can't configure the dma channel we want to fall
back to PIO. We do this by setting host->do_dma to zero.
This does not work as do_dma is used to see whether dma
can be used for the current transfer. Instead, we have
to set host->dma to NULL.
Signed-off-by: NSascha Hauer <s.hauer@pengutronix.de>
Cc: <stable@kernel.org>
Signed-off-by: NChris Ball <cjb@laptop.org>

drivers/mmc/host/mxcmmc.c

@@ -732,6 +732,7 @@ static void mxcmci_set_ios(struct mmc_host *mmc, struct mmc_ios *ios)
 				"failed to config DMA channel. Falling back to PIO\n");
 			dma_release_channel(host->dma);
 			host->do_dma = 0;
+			host->dma = NULL;
 		}
 	}