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>