mtd: nand: tango: Enforce DMA direction type

do_dma() uses an int to pass the DMA data direction information and
pass the same value to dmaengine_prep_slave_sg().

Currently, DMA_{FROM,TO}_DEVICE match DMA_{DEV_TO_MEM,MEM_TO_DEV}
definitions so it works fine, but assuming this will always be the case
is not safe.

Enforce enum dma_data_direction type in the function prototype and make
the enum dma_data_direction -> enum dma_transfer_direction conversion
explicit.

Reported-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>