mtd: Correct dependency of BLK

mtd: Correct dependency of BLK

In the case of MTD_BLOCK and UBI_BLOCK they should be select'ing BLK as
they provide block device functionality and not depending on some other
block device already being enabled too (as is the typical case).

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>