mtd: omap2: fixing compilation warning

Fixing below warning in compilation:
drivers/mtd/nand/omap2.c: In function 'omap_write_buf_dma_pref':
drivers/mtd/nand/omap2.c:508: warning: passing argument 2 of
'omap_nand_dma_transfer' discards qualifiers from pointer target type

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>