mtd: nand: denali: remove unused struct member denali_nand_info::idx

The struct member "idx" was used as an index for debug_array long
ago, but the DEBUG_DENALI feature was removed by commit 7cfffac0


("nand/denali: use dev_xx debug function to replace nand_dbg_print
and some printk").  Since then, this has been only initialized, but
never referenced.

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>