spi: spi-imx: use correct enumeration type

The fourth argument of dmaengine_prep_slave_sg needs to be of the
enumeration type dma_transfer_direction instead of dma_data_direction.
Since the used enumeration values actually stay the same, this is not
an actual issue at runtime.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Mark Brown <broonie@kernel.org>