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: Sascha Hauer <s.hauer@pengutronix.de>
Cc: <stable@kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>
diff --git a/drivers/mmc/host/mxcmmc.c b/drivers/mmc/host/mxcmmc.c
index 325ea61..8e0fbe9 100644
--- a/drivers/mmc/host/mxcmmc.c
+++ b/drivers/mmc/host/mxcmmc.c
@@ -732,6 +732,7 @@
 				"failed to config DMA channel. Falling back to PIO\n");
 			dma_release_channel(host->dma);
 			host->do_dma = 0;
+			host->dma = NULL;
 		}
 	}