Revert "mtd_blkdevs: don't scan partitions for plain mtdblock"

This reverts commit 776b54e97a7d993ba23696e032426d5dea5bbe70.

Looks like a last minute edit snuck into this patch, and as a result,
it doesn't even compile. Revert the change for now.

Signed-off-by: Jens Axboe <axboe@kernel.dk>