arm: mvebu: turris_omnia: fix leaked mtd device

After getting MTD device via get_mtd_device_nm(), we need to put it with
put_mtd_device(), otherwise we get

  Removing MTD device #0 (mx25l6405d) with use count 1

before booting kernel.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Pali Rohár <pali@kernel.org>
Tested-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>