mtd: nand: Only allocate ecc->{calc, code}_buf when actually needed

The only users of the ecc->{calc,code}_buf buffers are NAND controller
drivers implementing ecc->calculate() and/or ecc->correct(). Since the
->oobsize can be non-negligle, especially on modern NAND devices, we'd
better allocate it only when it is actually required.

Make ecc->{calc,code}_buf allocation dependent on the presence of
ecc->calculate() or ecc->correct().

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>