staging: mt7621-mmc: Replace dma dir with mmc_get_dma_dir

Currently the dma direction is manually determined by the read status,
there is a more portable function for it, mmc_get_dma_dir, use it.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>