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>