mtd: Fix mtd_check_oob_ops()

The mtd_check_oob_ops() helper verifies if the operation defined by the
user is correct.

Fix the check that verifies if the entire requested area exists. This
check is too restrictive and will fail anytime the last data byte of the
very last page is included in an operation.

Fixes: 5cdd929d

 ("mtd: Add sanity checks in mtd_write/read_oob()")
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>