mtd: nand: use usual return values for the ->erase() hook

Avoid using specific defined values for checking returned status of the
->erase() hook. Instead, use usual negative error values on failure,
zero otherwise.

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