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>

drivers/mtd/onenand/onenand_base.c

@@ -3694,7 +3694,7 @@ static int flexonenand_check_blocks_erased(struct mtd_info *mtd, int start, int
  * flexonenand_set_boundary	- Writes the SLC boundary
  * @param mtd			- mtd info structure
  */
-int flexonenand_set_boundary(struct mtd_info *mtd, int die,
+static int flexonenand_set_boundary(struct mtd_info *mtd, int die,
 				    int boundary, int lock)
 {
 	struct onenand_chip *this = mtd->priv;