mtd: nand: plat_nand: return error code of nand_scan() on error

The nand_scan() returns an appropriate error value when it fails.
Use it instead of the fixed error code -ENXIO.

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