mtd: nand: Fix nanddev_mtd_erase()

Commit e7bfb3fd ("mtd: Stop updating erase_info->state and calling
mtd_erase_callback()") removed the einfo->state field and the
MTD_ERASE_XXX macros. At the same time, the generic NAND layer was added
and made sure to update the erase info state.

It did not result in a build failure after merging the nand/for-4.17
branch in mtd/next because the generic NAND layer is not selected yet.
Let's fix that before a config option starts selecting MTD_NAND_CORE.

Fixes: e7bfb3fd ("mtd: Stop updating erase_info->state and calling mtd_erase_callback()")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>