The NAND ECC core is included in the generic NAND core when it is
compiled in.

Different software ECC engines drivers will select the NAND ECC core
and thus also have a dependency on the NAND core. Using a "depends on"
between the two leads to possible cases (not real cases, but created
by robots) where one is still unselected because of the "select does
not verifies depends on" game:

WARNING: unmet direct dependencies detected for MTD_NAND_ECC
  Depends on [n]: MTD [=m] && MTD_NAND_CORE [=n]
  Selected by [m]:
  - MTD_NAND_ECC_SW_HAMMING [=y] && MTD [=m]
  - MTD_NAND_ECC_SW_BCH [=y] && MTD [=m]

Fix this by using a select instead.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/linux-mtd/20201113123945.32592-1-miquel.raynal@bootlin.com