mtd: Hook I/O activity to the MTD LED trigger

Now that we've added the MTD LED trigger, we need
to call each I/O path to ledtrig_mtd_activity.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>