mmc: mxs: fix flags passed to dmaengine_prep_slave_sg

Since ceeeb99c we no longer abuse the DMA_CTRL_ACK flag for custom
driver use and introduced the MXS_DMA_CTRL_WAIT4END instead. We have not
changed all users to this flag though. This patch fixes it for the
mxs-mmc driver.

Fixes: ceeeb99c ("dmaengine: mxs: rename custom flag")
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Fabio Estevam <festevam@gmail.com>
Reported-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Tested-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>