i2c: mxs: use MXS_DMA_CTRL_WAIT4END instead of DMA_CTRL_ACK

The driver-specific usage of the DMA_CTRL_ACK flag was replaced with a
custom flag in commit ceeeb99c ("dmaengine: mxs: rename custom flag"),
but i2c-mxs was not updated to use the new flag, completely breaking I2C
transactions using DMA.

Fixes: ceeeb99c ("dmaengine: mxs: rename custom flag")
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>