mtd: onenand: Make flexonenand_set_boundary static

Fixes the following sparse warning:
drivers/mtd/onenand/onenand_base.c:3697:5: warning:
symbol 'flexonenand_set_boundary' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>