UBI: Fix trivial typo in __schedule_ubi_work

s/of/if/

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/wl.c

@@ -835,7 +835,7 @@ repeat:
  * @wrk: the work to schedule
  *
  * This function adds a work defined by @wrk to the tail of the pending works
- * list. Can only be used of ubi->work_sem is already held in read mode!
+ * list. Can only be used if ubi->work_sem is already held in read mode!
  */
 static void __schedule_ubi_work(struct ubi_device *ubi, struct ubi_work *wrk)
 {