spi, serial: move to dma_transfer_direction

fixup usage of dma direction by introducing dma_transfer_direction,
this patch moves spi, serial drivers to use new enum

Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Alan Cox <alan@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@iki.fi>