spi/spi-ep93xx.c: use dma_transfer_direction instead of dma_data_direction

A new enum indicating the dma channel direction was introduced by:

commit 49920bc6
    dmaengine: add new enum dma_transfer_direction

The following commit changed spi-ep93xx to use the new enum:

commit a485df4b
    spi, serial: move to dma_transfer_direction

In doing so a sparse warning was introduced:

warning: mixing different enum types
   int enum dma_data_direction  versus
   int enum dma_transfer_direction

This is produced because the 'dir' passed in ep93xx_spi_dma_prepare
is an enum dma_data_direction and is being used to set the
dma_slave_config 'direction' which is now an enum dma_transfer_direction.

Fix this by converting spi-ep93xx to use the new enum type in all
places.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>