mtd: rawnand: gpmi: drop unnecessary flag

The DMA_PREP_INTERRUPT flag is no longer needed by the mxs DMA driver,
drop it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>