mtd: change len type from signed to unsigned type

Callers of erase_write() always pass an unsigned int.
So this patch avoids a cast to an int.
Signed-off-by: huijin.park <huijin.park@samsung.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>

drivers/mtd/mtdblock.c

@@ -56,7 +56,7 @@ struct mtdblk_dev {
  */
 
 static int erase_write (struct mtd_info *mtd, unsigned long pos,
-			int len, const char *buf)
+			unsigned int len, const char *buf)
 {
 	struct erase_info erase;
 	size_t retlen;