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>

drivers/mtd/nand/omap2.c

@@ -505,7 +505,7 @@ static void omap_write_buf_dma_pref(struct mtd_info *mtd,
 		omap_write_buf_pref(mtd, buf, len);
 	else
 		/* start transfer in DMA mode */
-		omap_nand_dma_transfer(mtd, buf, len, 0x1);
+		omap_nand_dma_transfer(mtd, (u_char *) buf, len, 0x1);
 }
 
 /**