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>
diff --git a/drivers/mtd/nand/omap2.c b/drivers/mtd/nand/omap2.c
index 1bb799f..4eea97c 100644
--- a/drivers/mtd/nand/omap2.c
+++ b/drivers/mtd/nand/omap2.c
@@ -505,7 +505,7 @@
 		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);
 }
 
 /**