mtd: nand: denali: Avoid using ecc->code_buf as a temporary buffer

ECC bytes are contiguous in the ->oob_poi buffer, which means we don't
have to copy them into ->code_buf (here used as a temporary buffer)
before passing them to the nand_check_erased_ecc_chunk() function.

This change will allow us to allocate ecc->{code,calc}_buf only when
ecc->calculate() or ecc->correct() is specified.

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