dmaengine: mxs: rename custom flag

The mxs dma driver uses the flags parameter in dmaengine_prep_slave_sg() for
custom flags, but still uses the dmaengine specific names of the flags.
Do a little bit better and at least give the flag a custom name.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>