mtd: onenand: kill unused variable

Fix this gcc warning:

drivers/mtd/onenand/onenand_base.c: In function ‘onenand_block_markbad’:
drivers/mtd/onenand/onenand_base.c:2636:23: warning: unused variable ‘this’ [-Wunused-variable]

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>