mtd: OneNAND: Fix onenand_block_markbad

commit dfe64e2c
    Author: Sergey Lapin <slapin@ossfans.org>
    Date:   Mon Jan 14 03:46:50 2013 +0000

        mtd: resync with Linux-3.7.1

modified onenand_block_markbad to call mtd_block_markbad,
but as _block_markbad function pointer used by mtd_block_markbad
to do actual job is by default pointing back to
onenand_block_markbad there is no way this function ever
finishes its job.
Fix it by changing function body according current (4.12-rc6)
linux implementation.
Tested on IGEPv2 board with Muxed OneNAND(DDP) 512MB containing
several unerasable blocks this function marked bad.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>