mtd: nand: fmsc: kill {read, write}_dma_priv from fsmc_nand_platform_data

The read_dma_priv and write_dma_priv fields of fsmc_nand_platform_data
are never set, so this commit removes them.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>